when a unit has assault 4, is the number fixed? or will it stack when it attacks again?

No — printed Assault 4 doesn't grow on later attacks. It's a passive ability that reads "While I am an attacker, I have +4 [M]," so it switches on for each combat you attack with it and switches off when it stops being an attacker. Attack twice in a turn and it's Assault 4 both times, not 4 then 8.

Assault only accumulates when the unit has multiple instances of the keyword from different sources — a printed Assault 4 that also gets hit by two Cleaves (Assault 3) is Assault 10 for that combat (Rule 807.2, FAQ 11782). Those are separate "this turn" grants stacking on top of each other, not a single static value repeating.