I am currently building a 1/5th scale Corsair from plans. The plans depict an F4U-1A but wanting to build an F4U-4. I am aware of most of the differences between the two models, mostly front air scope, 4 bladed prop, different cockpit etc. Are there any major structural differences? Are the fuselage and wings the same shape and size? I have tried to compare pictures and the outlines look identical as far as I can tell.
Engine cowling as you mentioned, the cockpit was different internally, and there were minor differences in the tailplanes and vertical stabilizer, they look slightly "beefier". The cowling differences were mostly a result of moving the air intakes from the leading edges of the wings to the cowling itself.
I would check on the various blueprint and drawing sites for some comparison drawings just to be sure.
I have a buddy rebuilding the 11th from the last FG1D built (with F2G airframe mods) which is remarkably complete and original except the left wing is tagged as F4U-4.