I don't remember, was Enterprise officially renewed when "Zero Hour" was in production? My gut says "no" because I was shocked they went with a cliffhanger. One of my biggest TV bugbears are shows on the bubble that insist on cliffhangers for their season finales. Some producers actually go on record as saying "this will force the network into a renewal" which doesn't f'n work. Too many shows I really enjoyed ended without resolution, such as"Nowhere Man," "Alphas," "The Fugitive" (2000-2001), the "V" remake (which I didn't actually "enjoy" but I was very interested in seeing how certain characters dealt with the cataclysmic turns in the finale), "Earth 2" and so on. Leaving Enterprise in that "look SPACE NAZIS!" finale would have been a greater disservice that fans would be bitching about to this day - probably worse than what we were given. As poorly as the actual finale was received, at least it was a finale.
Having said that, I do also hate finales that split up and/or kill off charaters. DS9 was heartbreaking, but nothing ripped my soul out as much as Babylon 5 - even though those were beautiful tears. And plenty of stuff could happen between the last two episodes.