I could see them cutting out the Moon trio buying the caleidoscopes and just make pegasus give them these. Also maybe the amazon trio transformation might be pushed into Ami's epsiode or split into three parts (basically each animal being transformed in the epsiode of their mission). Probably also less fluff between Usagi, Chibiusa and Mamoru in his apartment
If they do things like adjusting the beginning and ending of each act, then it might end up being fine.
But they will probably have to do a recap of the end of Act 38. I know people gave Season 3 flack for doing recaps, but people need to remember that it's not just superfans that are watching the show now that it's airing on TV and a lot of time will have passed since Season 3 ended.
That and with so many different scenes; Rei and the others saying their dreams, Usagi and Mamoru at the circus, then at their apartment, Chibiusa's dream riding Pegasus, Chibiusa finding out she can't time travel, introduction of the enemies, new transformation, new attack, switching ages, etc. Even if you cut out certain things, they will end up with a very rushed first episode which sounds like a really bad idea for the start of a brand new season (especially when it seems unlikely that they can get away with only 11 episodes). They didn't even cover Pegasus' first appearance in Infinity's finale... There's just way too much to cram I think.
Of course, you're right that if they do a two part episode, then it would mean the season premiere would not have a single appearance of a Sailor Soldier... But perhaps ending the first part with the girls realizing they can't transform might seem like a juicy cliffhanger for the start of the fourth season. I guess it's up to the director in this case. But if they did a two part first episode for Season 3, I don't see why it's suddenly unlikely for that to happen in Season 4.