Director of Adventure Management Operations
Fiberglass kinda makes that irrelevant. Wooden boats spend almost 100% their time getting pounded while on the water and they don't use 4" wide timber for much other then the keel.I think the 4x4s would work better to resist shear, with the greater surface area than 2x3s, and especially when two plywood sheets are butted up against each other and glued-and-screwed to a 4x4.
Also, if you put liquid nails on a standard truss top cord and screw down the plywood roof decking, the screws become irrelivant once the glue dries. You won't be able to remove the plywood with out completely destroying/delaminating it. Once the glue bonds, it's not letting go.