Comparisons
UnderCover UltraFlex vs BAKFlip MX4
Nearly identical on paper. One of them is rated to be driven with the cover folded open, and that is the whole decision.
By Michael W. · Published October 5, 2026
Cover & Cargo earns a commission from qualifying purchases made through links on this page, at no extra cost to you. It does not change what we recommend or the order we rank things in — here is how we choose and here is the full disclosure.
What this is based on: published manufacturer specifications and warranty terms, cited on each cover; independent published tests where they exist, named and linked; and verifiable owner reports of specific failure modes. We have not fitted these covers to a truck, we do not run a test lab, and we do not claim a test we did not run. Where the evidence for a cover is thin, the page says it is thin.
Quick picks
| # | Cover | Best for | Score | Price |
|---|---|---|---|---|
| 1 |  UnderCover UltraFlexThe folding cover built to be driven open: it folds to the cab and the maker states you can drive that way, which is the single most useful thing a folding cover can offer a person hauling tall loads. | If loads are tall | 8.4out of 10 | |
| 2 | BAKFlip MX4The default hard folding cover, and the one most buyers should start from: 400 lb rating, a 5-year warranty, 100 percent bed access, and a seal design the maker revised specifically to stop bulkhead seepage. | If they are not | 8.4out of 10 |
Scores are our own editorial judgment on the rubric published at our editorial policy page, weighing security, weather sealing, bed access, install effort, durability, value. They are read off published specifications, warranty terms and sourced owner reports. They are not measurements we took, and they are not user ratings.
The short answer
Both: 400 lb, 5-year warranty, 30-minute no-drill install, matte black. On the spec sheet these are the same cover.
The UltraFlex is published as safe to drive with the cover folded open. The MX4 makes no such claim, and neither does anything else in our registry.
Read UnderCover's towing clause before you buy: do not tow the vehicle from the rear with the cover on.
How similar they are
| UnderCover UltraFlex | BAKFlip MX4 | |
|---|---|---|
| Load rating | 400 lb, evenly distributed | 400 lb, evenly distributed |
| Warranty | 5-year limited | 5 years, not transferable |
| Install | 30 minutes or less, no drilling on most | 30 minutes or less, no drilling on most |
| Construction | Aluminum with a high-density polymer core | Multi-layer aluminum, polymer-reinforced tops |
| Finish | Matte black | Textured matte black |
| Bed access | 100 percent all the time - rated to drive folded open | 100 percent, folds flush against the cab |
| Published caution | Do not tow the vehicle from the rear with the cover on | None published in the specification |
These are two of the closest-matched covers in this registry. The differences below are small except for one, and that one is in the bed access row.
The difference that matters
UnderCover publishes the UltraFlex as giving “100 percent bed access — all the time”, folded to the cab, and states you can safely drive with the cover fully open.
No other cover in this registry makes that claim, and the absence is not an oversight. A folding cover driven open is a large flat panel standing upright in an airstream, and most manufacturers would rather not warrant that. UnderCover does.
If you ever carry a refrigerator, a stack of lumber, bikes upright, or anything else taller than the bed rails more than once or twice a year, that single published capability is worth more than any other difference between these two covers.
The clause to read first
UnderCover's published warranty text states: do not tow your vehicle from the rear with the cover on. If you ever flat-tow or dolly the truck from the rear — a real scenario for anyone who tows a vehicle behind an RV, or who has had a breakdown — read that clause in full before you buy.
It is specific, it is published, and it is the kind of thing nobody notices until it matters.
The verdict
Buy the UltraFlex if tall loads are part of your life. The driven-folded rating is the only thing on this page that is genuinely unavailable elsewhere.
Buy the MX4 if they are not. It matches on rating, warranty length and install time, publishes a revised seal design aimed specifically at bulkhead seepage, and comes without the rear-towing caveat.
If you want the tailgate seal instead
UnderCover's ArmorFlex publishes a dual-action tailgate seal that lets the tailgate drop without unlatching the cover. Same 400 lb and 5-year terms, with original-owner registration required — on the folding covers page.
The picks in detail
#1 · Carrying tall loads often
UnderCover UltraFlex
UnderCover · Hard folding
The folding cover built to be driven open: it folds to the cab and the maker states you can drive that way, which is the single most useful thing a folding cover can offer a person hauling tall loads.
What it does well
- Rated to be driven with the cover folded open — most folding covers simply do not say this
- 400 lb evenly distributed rating with a 5-year warranty behind it
- Polymer core gives the panel more dent resistance than a hollow aluminum skin
Where it gives way
- Folded to the cab it still consumes bed length, driving or parked
- UnderCover's warranty specifically rules out rear-towing the vehicle with the cover on — read it before you flat-tow
- Matte finish needs a dedicated protectant rather than wax
Skip it if: Towing from the rear with the cover fitted — UnderCover's own warranty says not to.
| Construction | Aluminum with a high-density polymer core |
|---|---|
| Finish | Matte black |
| Load rating | 400 lb, evenly distributed |
| Warranty | 5-year limited |
| Stated install time | 30 minutes or less |
| Drilling | None on most applications |
| Bed access | 100 percent, all the time — rated to drive folded open |
Specifications read from RealTruck — UnderCover UltraFlex (Gen 3) published specifications.
#ad We show no price until we can verify one. How this is funded
#2 · Most truck owners
BAKFlip MX4
BAK Industries · Hard folding
The default hard folding cover, and the one most buyers should start from: 400 lb rating, a 5-year warranty, 100 percent bed access, and a seal design the maker revised specifically to stop bulkhead seepage.
What it does well
- 400 lb evenly distributed rating is at the top of the folding class, and it is published rather than implied
- The revised EPDM perimeter seal plus drain system targets the bulkhead, which is where folding covers actually leak
- Folds flat against the cab for full bed access without taking the cover off the truck
- 5-year warranty is longer than most hard folding covers in the same bracket
Where it gives way
- Folded open it occupies roughly the front third of the bed — the published 100 percent access means the bed is reachable, not that the space is free
- Textured black finish shows water spotting on dark trucks more than a gloss panel does
- No published figure for cover weight, so the one-person-install question is unanswered by the maker
Skip it if: Anyone who needs the bed open and covered at the same time all day. Folded against the cab it eats the front third of the bed, and nothing folding escapes that.
| Construction | Multi-layer aluminum, polymer-reinforced top panels |
|---|---|
| Finish | Textured matte black |
| Load rating | 400 lb, evenly distributed |
| Warranty | 5 years |
| Stated install time | 30 minutes or less |
| Drilling | None required on most trucks |
| Bed access | 100 percent — folds flush against the cab |
| Sealing | EPDM perimeter seals, integrated drainage |
Specifications read from RealTruck — BAKFlip MX4 (Gen 3) published specifications.
#ad We show no price until we can verify one. How this is funded
Common questions
What is the difference between the UnderCover UltraFlex and BAKFlip MX4?
Almost nothing on the spec sheet — both publish 400 lb, a 5-year warranty and a 30-minute no-drill install in matte black. The one real difference is that UnderCover publishes the UltraFlex as safe to drive with the cover folded open, which no other cover in our registry claims.
Can you drive with a tonneau cover folded open?
On the UnderCover UltraFlex, yes — UnderCover publishes it as giving 100 percent bed access all the time and states you can safely drive with the cover fully open. Most manufacturers make no such claim, which is why you should not assume it for other covers.
Can you tow a truck with an UnderCover cover fitted?
UnderCover's published warranty text says not to tow the vehicle from the rear with the cover on. If you flat-tow or dolly the truck from the rear, read that clause in full before buying.
Which cover seals better, the UltraFlex or the MX4?
BAK publishes a revised EPDM perimeter seal and tight-fit clamp design on the MX4 aimed specifically at seepage between the cover and the bed rails, plus integrated drainage. UnderCover's published specification for the UltraFlex is less detailed on sealing, so the MX4 has the better-documented case.
Sources
Read next
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UnderCover Tonneau Covers: Review and Known Problems
The only cover rated to be driven folded open, a dual-action tailgate seal, and the rear-towing clause to read before buying.
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BAKFlip Tonneau Covers: Review and Known Problems
BAK's range, its warranty exclusions, and the side-rail leak and latch complaints owners have documented in public threads.
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BAKFlip MX4 vs BAKFlip G2
The clearest upgrade decision in the category: 100 lb of rating and two years of warranty, for a price difference.
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