Skip,
Frames are pretty close to each other. Biggest difference I can think of right now is the way they attach to the WS frame. The TR4a has a luggage type latch, where the TR6 has pins that go into the frame from the top. I have put a TR6 convertible top on a TR4a frame and it worked fine. Years ago I bought a TR4a that didn't have a top frame. I had an extra tr6 top frame and WS. Changed them out and it all fix like from the factory.
Marv