You really can't get too much shorter on a cat without that feeling cramped.

Our Mono is a 43' Beneteau. 3 cabins, 3 heads. A 40' cat gives us an extra cabin and all cabins are roomier. It has fewer heads at 2 but over all has a lot more space.

So it really depends on what you want. Get a little bigger and everyone can have their own private head. Smaller and you are likely sharing heads. Given there will only be 4 of you, I'd probably go with something around 40'.