Jack,
I re-upholstered the Speedster type seats that I have. I made the upholestry for eash seat as 4 pieces as you suggest.
I couldn't get the CMC 1 piece back/sides to wrap without lots of wrinkles (plus they were the wrong color). That's why I made my own.
I made the bottom seat cushion as 1 piece. The back cushion is one piece & attached to the fiberglass bucket at the top and bottom of the cushion only. Each side is a separate piece and attached per the CMC manual. (Could have used glue only, but opted for wood strips and staples)
Greg