The only real 'limit' is in terms of exotherm, which then depends on your resin. That's also very thick solid laminate.
In this case there is no real limit, it's just unnecessary. Of course if you have a part that absolutely needs to be 4mm for fitment reasons then you need to make the part 4mm. But the advantage of composite parts is you can be selective. If you have a mating surface that needs to be 4mm, you can make that section 4mm but the rest of the part can still be 1mm.
Really it just takes a lot of material to make a 3-4mm part. If it doesn't need to be that thickness for fitment reasons, and it is only a cosmetic part, then it's just a waste to make it so thick.