Zygos 4.0 Hydration Pack

(13 customer reviews)


Bryce Thatcher’s pack of choice, the Zygos 4.0 hydration pack is very versatile. This pack is lightweight and high capacity, a comfortable wrap around style with full compression allow for the lowest profile empty or fully loaded. The Zygos 4.0 comes with a 2L reservoir and a magnetic clip to keep your hands free. New expandable mesh on the back panel gives 4 liters of extra capacity as well as storage in the front with extra pockets. Whether you are fastpacking, mountain biking, in a fast race, ultra race, or summit assault this hydration backpack is the solution for any athlete. Now available in five sizes.





Race Line: Built for the competitive athlete. Our Race Line focuses on Ultra & Trail running but can be used in any running scenario. Our Race Line products are sized to fit the athletic body, feature UltrAcool technology for maxiumum breathability on all race vests, made with very lightweight material for comfort during long distances, and created for pure performance.

  • Recommended Use:
    Versatile Pack utilized in any run scenario, training or racing.


    • Lightweight, soft, breathable, but strong, large-holed mesh pulls moisture away from the body to keep skin dry and cool.
    • Silicone and PU coated feather-weight nylon fabrics resist water, abrasion and tears.
    • Max O2 Sternum™ for unrestricted breathing while maintaining a stable pack. Uses shock cord with good stretch for unrestricted breathing but also good rebound on the stretch for a stable ride. Patent Pending.
    • Plush feeling micro-fiber polyester edges, soft and non-abrasive against skin and clothing.
    • Sweat proof webbing—won’t absorb water or get saturated and heavy.
    • Safety stitched bar tack on all stress points.
    • Quiet, bounce free, easy to see and use, zipper pulls.
    • Zig zagged stitching over elastic for ultra-durability, increased stretch and rebound.
    • Simple, fast, easy in and out bladder attachment.
    • Magnon pocket for sweat-free pills, with automatic magnetic closure for rapid access to pills and other essentials. Patent pending.

Fit:  Unisex
Size(s):  2X Small (26″-32″) X Small (28″-34″) Small (30″-36″), Medium (32″-38″) or Large (34″-42″)
Color:  Pitch Black, Emerald Blue*
Hydration (included):  2L Reservoir with mag-clip to secure hose.
Bottle Compatible (not included):  UltraFlask, Formula 250, Human 20 oz 2.0, or Collapsible Bottle
Capacity: 854 c.i. (14L)

Additional information




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Fluid Capacity

2 L (70 fl. oz) Reservoir

Total Volume

14 Liters

Weight (Includes Reservoir)

16.0 oz (454g)


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13 reviews for Zygos 4.0 Hydration Pack

  1. Jake Grossbauer

    Took the Zygos 4.0 on a test run and after first thought i am happy with purchase.
    Bought it for future all day or even overnight fastpack runs hoping the storage would be enough.
    For grins i tested the underarm pockets which i was able to easily fit multiple gels, waffles, and other nutrions in. The added front pockets allow for easy access to phone pr map while still having bottles in each as well.
    The fit is true to size amd the honeycomb wicked perfectly as it was warm bit the pack did not absorb any.

    The underside zipper pocket on pack was easily accessible and perfect storage for when it warmed up to put buff and gloves in, while never breaking stride

  2. Matt Ozanic (verified owner)

    This vest is AMAZING!! I’ve had many vests over the years but this one beats them all, hands down. I love the placement of pockets, materials used and the overall thought of design that will be long lasting and comfortable at the same time. It is the fine-wine of hydration packs. This pack is going to be great at keeping my fluids cool on my training runs in western Colorado and it will carry all of my needed gear on High Lonesome 100 and The Bear this year. Thank you for all that you put into this pack. It is absolutely perfect!!

  3. Sean Haverty

    I got the Zygos 4.0 after Long time use of UD vests. First impressions right out of the box was Quality. The mesh and fabrics used are seriously the finest I have seen.
    Skip to use: the Zygos 4.0 come last with a insulated bladder pocket. This is huge for me since I seem to get the most refreshment from cold water, and this allows it to stay cold for a long time. Special bonus; the bladder closure is attached to the bladder itself! More than once did I forget or lose the closeure on my UD bladder rendering it useless.
    The fit: at first it seemed a little strange that the vest didn’t fit snug on my shoulders, yet hung above them a bit. However after miles of use now I found that this is a blessing. It’s like wearing nothing at all!
    The pole straps are different in that they are located at the bottom back of the pack. I thought I would have a problem easily accessing these but I was pleaseanlty supprised. I can take them out and put them back in with ease, while continuing to run.
    Then there is the magnetic drinking tube clasp. So convienient and quick. My only problem was that I felt I would lose the receiving magnet that attaches to the vest on a clip. I solved this by attaching the clip to the top vest closure bungee. It doesn’t come off of there unless you want it to.
    Next is storage space. There are plenty of pockets and places to put anything you need to carry, without a problem.
    I really am impressed with this vest. If you are thinking about it, pull the trigger and buy it. You won’t be disappointed.

  4. Sean Haverty

    After being a UD vest user, I ordered the Zygos 4.0, mainly because of the support that UltrAspire gives to everyday mortal runners, unlike UD.
    First impressions: out of the box the first thing I noticed was the quality. The mesh and fabrics used are such high quality.
    Let’s get to the use: The bladder is easy to fill, and it has an attached closure! I can’t tell you how many times I have lost or forgot the closure on the UD vests, rendering it completely useless.
    The bladder pocket is insulated which I love as I feel more refreshed with cold water and this keeps the water cool for quite some time.
    The fit of the vest for me was at first unusual. Perhaps it’s because I have a big frame, but the shoulders seemed to rise above a bit. After many miles now, it’s actually a godsend. It’s like I’m not wearing anything at all.
    The pole straps are located in the bottom back. I thought I would have trouble reaching and using these but so was pleasantly supprised. I can take my poles out and put them back with ease even while running.
    The storage space is massive, the pockets are easy to reach.
    Did I mention the magnetic drinking tube holder? This is really awesome and fast. At first Inwas afraid I would lose the attachment that goes on the vest with a clip, but I remedied this by attaching it to the top vest bungee closure. It doesn’t come off of there unless you want it to.
    I love the bite nozzle on the bladder. It delivers water with no effort.
    If you are undecided on this product, go ahead and pull the plug. You won’t be dissatisfied.

  5. Sascha Jurak

    I used it on my last marathontrail and it is the best of the best.
    Everything is very good reachable and the pockets have the perfect size. The only thing i don´t like to much is the position of the poles. I have to put the Zygos of my back to put the poles in and out. And sometimes when i run downhill i hit the poles with my elbows. Mybe there is a possibility to clip and add on in front to sstre the poles there?

  6. Joshua Hamilton

    First off, I am an Ultraspire Immortal Ambassador so I currently only use Ultraspire packs. In the past I have primarily used Salomon gear and I’ve always been a big fan of their stuff for all mountain activities. This pack is one of the highest quality pieces of gear that I have ever used….period. Phenomenal fit and finish with excellent features that all seem to work seamlessly together. The materials are lightweight and super high quality. Ultraspire has definitely outdone themselves with this one. I absolutely love it.

  7. Jeff Fullmer

    Like many others, I was a diehard UD fan. I was shocked to take this out on my first run and realize I could breathe in it! With previous vests from other companies, I constantly have to adjust the chest straps to breathe more on the uphills and tighten it up on the downhills. I have yet to adjust the chest straps mid-run with the Zygos 4 as it breathes with you! I LOVE the lower pouch on the back of this pack. It has my tp and first aid kit easy to access but out of the way. Most runs I don’t use them but when I do its nice to not even have to take my pack off to access this small tucked away lower compartment. I LOVE the waterproof pockets like the magnetic pill pouch, cell phone, and interior pack pockets. The all-day comfort is huge for me as well. I don’t ever feel like this digs into my shoulders. I can carry A LOT of gear for all-day mountain adventures (and enough emergency supplies to make it through a night if I had to) or just throw my cell phone in it for a quick morning run. The versatility is astounding. Open access to the water blatter compartment and insulation are a big bonus as well. The quality is outstanding and meticulous; from the bungees to the stitching, straps, and pockets. I foresee this being a 10 year+ pack and will probably leave it in my will for my favorite son. HA! 🙂

  8. Aaron Smith (verified owner)

    Zygos 4.0: awesome pack! I took it on two 50k’s and it stored everything I needed, I probably took a little more than I needed. The races were supported with aid stations at decent intervals so I decided not to take the included 2L bladder but I did take two 500ml Softflask bottles (sold separately) with me each time.

    For my solo training runs, however, I have taken the bladder, both bottles, and fully-loaded the pack with additional weather-dependent layers and emergency gear. I love the many pockets and they store everything I need (keys, phone, ID, clothes, gloves, hat, food). Everything can be accessed very easily while moving so I can grab a snack from a pocket without slowing down (too much!).

    Quality seems to be solid, I have fallen a few times and it hasn’t torn or ripped. I enjoy the magnetic clip of the 2l bladder, I love the various features such as the bungee system that tightens the pack as needed or the clips/zippers of the pockets. Everything is very appropriately placed, you can tell that a runner/adventurer designed this with other runners/adventurers in mind.

    Great pack!

  9. Tim

    This is a very low profile race vest despite the storage capacity which is a good thing! This vest is ideal for self support long runs in the mountains or races in which a mandatory gear kit is required. My one purpose of buying this vest was to test that specifically. I have used the vest twice with the gear kit requirements for UTMB on an 5 hour and 8 hour mountain run. I have never used the bladder as I do not like using them and refer the flasks as the are more versatile/quicker at aid stations. The vest was excellent on both runs, no movement, everything was accessible when needed. I did have issues storing poles as they hit my elbows and arms when running. My preference would be for UltrAspire to look at the ability to store poles vertically vs horizontally. Other than that one suggestion the vest has everything one would need for kit specific races. One suggestion is put the side pockets that are on the Zygos 4.0 on the Momentum.

  10. Holly

    This is my new favorite all-purpose hiking and trail running pack. I can fit everything I need for many different kinds of adventures and can distribute my gear weight and bulk throughout the many pockets so that I don’t feel weighed down. I love that my chest does not feel constricted with this vest – breathing is easy with the flexible straps. It is easy to adjust while running as my pack looses (or gains) volume with gear. The back pocket fits my puffy jacket and rain shell nicely and it feels secure (not bouncy).

    My suggestions for improvement are: have more size choices (I need the small normally, but the medium when wearing a jacket or a padded sports bra), remove the foil liner for the bladder (it’s noisy, hot, and doesn’t seem to add value), and consider a more secure clip design for the bladder straw magnet (I need to consciously remember to pull down to release it from the magnet when I want to drink, even though my instinct is to pull up and towards my mouth).

  11. Danny Widerburg

    I’ve ran in quite a few vests over the years, I seem to have always come back to my UltrAspire Zygo’s. I had the original for several years, it never failed me. Recently I acquired a Zygo’s 4.0 and the best compliment I can give it,
    “I never notice that I’m even wearing it”. It carries what ever I need for the current adventure, its quality that you can see and feel and it fits correctly. Hands Down, the BEST Running Vest I have ever used!

  12. Ron Anderson

    If Tony Stark made hydration vest, this would be it. I love a good piece of kit and Ultraspire did not disappoint. First rate materials and craftsmanship with plenty of pockets/storage for your ultra mileage arsenal. Look forward to sharing more miles with this vest.

  13. Olan Johnson

    I have been running with the Zygos 4.0 for a few months now, but wanted to see how the pack performed during my first 100 miler before posting a review. The bottom line up front is that this is a must-have piece of gear if you are taking on the 50 mile or 100 mile+ distances.

    I love that the low-profile design allows the vest to wrap around you in such a way that it feels like a part of your body instead of another layer that you are wearing. As a result, the vest is super comfortable and guarantees multiple solid points of contact. This means that I was able to wear this vest – fully loaded – for 29.5 hours straight without experiencing any fatigue or chafing. For reference, my version of fully loaded means carrying a full 2L water reservoir, 1x 550 UltraFlask, phone, running fuel, blister kit, first aid kid, additional layers of clothing, and a headlamp. I’ve also been impressed with how secure yet adaptable the Max O2 sternum system is. With previous vests, I would constantly have to adjust and re-adjust the straps throughout my run. The Max O2 system eliminates that completely. I can honestly say that I have yet to re-adjust the shock cords on the go once I’ve set them to my liking.

    Another major reason I love the Zygos is all the massive amounts of storage space! The pockets on the front (2 on each side) are deep and spacious. No zippers up front makes for easy grab and go whether its fuel or flask bottles. Another favorite aspect of this vest are the side pockets because they span the entire side portion of the vest. I’ve stored everything from fuel to headlamps to cold weather layers in these pockets, something I could not do comfortably – or at all – with other brands. One storage space I had not used until my 100 miler was the pocket at bottom of the pack. The placement seemed strange to me and I wasn’t sure how I’d use it. But this is where I ended up keeping my gloves and I can’t tell you enough how awesome it was to quickly access and store them here at various points throughout the race. The only aspect of this vest I haven’t been able to test yet is trekking pole storage. But I’ll be sure to check back in once I’ve had an opportunity to utilize them!

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