My sense is that 4mm lends a more rigid structure and 3 mm with the amount of glass making up the weight difference makes a more flexible laminate with less catastrophic failure. Don't think there would be any practical savings going from 4 to 3 with more glass on the latter. I could imagine going from 4mm to 3mm then using s-glass. Using 4oz s-glass on a 4mm test panel I could get the 8" panel to bend into 1/3 circle without a total fracture whereas the 6,4oz e-glass would fracture (bent parallel to grain) through the wood right along a set of glass threads and wood grain. Of course the 6oz e-glass was more rigid and failed at a higher load but the nature of the failure was the same.