What size barrel should I run?

Hello folks, new here. Forgive my ignorance and if this has already been asked a lot but

I'm about to build my first 9mm AR pistol, I'm putting a rugged obsidian on it..what barrel length does everyone seem to think works the best with running subs? I was thinking of going 4 in for compact ability but I've heard and seen that 8 in works really well with subs.. just looking for some advice or tips. Also probably doesn't help but it's going to be a AERO build with a FRT trigger

Thanks. Cheers and happy new years