4x4 is the standard for pre preg, if its dry fabric you will be using be wary that it is a nightmare to work with due to its loose characteristics, also its a heavy weight.

If its prepreg (low or high temp) it will be fine, 2 layers of 300g should see a thikness of around 5mm.

We use 3 layers on most items like this, at the mo we have just finished the moulds for rear floor pans for the escort mk4, these will be made from 3 layers of 200g, infused with a thikness of .7mm.

it all depends on the purpose of the panel, if cosmetic and just to replace metal to save weight with no structural integrity required then less if obviously better but if the panel is going to be subjected to harsh conditions such as a full aerofoil style under side then I would either go with a carbon/aramid/carbon stack of the 4x4 you mentioned.

Experiment with a few small panels unless someone else is making them of ourse which could add cost for no reason, but in that case their own expertise should be enough to decide on the correct wy to go.

Any probs feel free to give us a call, working late all this week, got a twin custom gauge pod to finish for a fruity corsa :-( lol