Planning to make a pulley cover then it dawned on me that I have to take temperature in to consideration.

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.