All P4 modules (for the Big Time, Classic, and Executive) are dimensionally identical and fully interchangeable. That being said, there are only certain combinations that are "legitimate", in that they were offered by Time Computer as part of the product line.
There were four different P4 modules; 401 (magnet-set), 402 (Auto-set), 403 (Day-of-Week, Auto-set only), and 406 (24HR display, Auto-set only). The factory combinations are:
401 All P4s
402 All P4s
403 Classic, Big Time
406 Executive only
So, if you see an Executive with a 403, or a Big Time or Classic with a 406 (which I have seen, on eBay), you can be very sure someone did some module-swapping somewhere along the line, and the watch is no longer completely original. Still original TC modules, just in the "wrong" case.