I’m assuming from what you’ve been doing that 1) you think your acts should all have about x number of words and/or 2) you think all acts in a book need to have about the same number of words to feel balanced. So, given that, could you, hypothetically, split act 2 into two acts and have five acts instead of four? . . . . Is there a rule of thumb about the number of acts one can have in a book like yours? I feel like I’ve seen different numbers of acts used in different literary works but perhaps I’m missing something obvious due to the fact that this is not the kind of writing I do.
There is no rule on acts.
They’re just a tool that I use to structure. So here’s my theory of acts–ust mine, nobody else’s, not a rule, merely a cheat sheet for me, the lousy plotter: