The Army used M4's, M4A1's and M4A3's. The difference is in the engine installed, the engine deck, the exhaust system and the rear hull. In these aspects, the M4 and M4A1 Shermans are quite different from the M4A3 Shermans. The engine differences are a moot point unless you want to show the engine compartment. The M4A3 has grills on the top of the engine deck, where the M4/M4A1 does not. The exhaust systems are totally different from a M4/M4A1 and a M4A3. As a result of the different engines, the rear hull's were different between the M4/M4A1 and the M4A3. The M4/M4A1 had two filter cans on the rear hull and a larger engine access door than the M4A3. The M4A3 had an exhaust shield which the M4/M4A1's did not. The M4 and M4A1 Shermans have the same engine, but the M4 has a welded hull and the M4A1 has a cast hull.
The composite hulled M4's had a cast front hull like an M4A1, but a welded rear hull like a standard M4. They had the same engine as the M4/M4A1.
You are correct on the Flamethrower tanks on Okinawa being Army and not Marine. I was far from my reference material when I answered origianally. Thanks for the correction.
I hope this helps answer some of your questions. Good luck no matter what you decide to build.