The only drawback about the 4-door A-car is that they are not sought after by collectors, and have low resale value. The only drawback is that the aftermarket may not stock parts that are reproduced for the 4-door (e.g. weatherstripping, full quarters). Unlike the hardtop, pickup, and convetible, the 4-door and wagon are seldom looked at.
A few parts can be used, like the dash, front clip, suspension components, drivetrain exc. driveshaft, rear deck, taillight housings, rear seat, and bumpers. The front seat tracks can be used, since the seat is the only thing that differs.
Other things that can be of some use are the door handles, window cranks, and lock cylinders.
The frame, doors, glass, and body? They have no use, unless someone wants to build a 4-door SS clone.