Since I didn't really get a response on my questions I'll give my opinion on the battle then: No failsafe for Ben: Four Screeches, Ben likely detransforms and then gets yeeted.
Failsafe for Ben and Goop has High regen: Big Chill and Ghostfreak have feats of phasing through lasers and radiation, 2 non-physical things so there is no guarantee Four's non-physical interaction will work, Atomic-X can teleport and Nanomech can become too small for him too hold. In short no incapacitation for Four.
Four's BFR: If Four astral projects then the battle continues as usual, if he does not then Ben will use his supergenius intellect with Brainstorm and Grey Matter and his building capabilities with Juryrigg and Upgrade to make something that can bust him out of there. In short no (useful) BFR for Four.
Four killing Ben: The Omnitrix' failsafe is capable of reacting to the Big Bang and will pick whatever alien Ben needs to survive the current situation. This could be Goop with his High regen to counter the AFB or just switching Ben into another alien to counter biological manipulation and possibly transmutation (I don't think it's in character for Four, correct me if I'm wrong). In short no killing for Four.
That leaves Four with one option: KO: Now Four has a decent AP advantage, (although Atomic-X probably makes the advantage negligible via upscaling) but he lacks combat skill compared to Ben who would literally skillstomp him into the ground with Master Control at age 46, in fact
(14:20) I'd say 10-year old Ben with Master Control skillstomps Four and Ben 10000 has 3 decades more experience than that. Oh and he can speedblitz with XLR8 and since Four has Low-High Regenerationn that's allowed.
Ben also has hax such as sleep manipulation and life-force absorption (not very in-character) which could instantly win him the match and has sealing with Null Void Eggs (he has to detransform to do this but his speed when detransformed is still FTL and thus equalized to Four and all he needs to do is throw them) which is in-character for him to use.
In short, I vote for Ben.