Nice job. I have been thinking why hasn't someone come up with an adaptor that would screw onto the P3 motor, but at the top of said adaptor, have the quick disconnect for p4 props. It would raise the props, but I don't really see how motor/prop height is critical.
My first thought on your project is, forget trying to get the motors apart. You see how difficult it would be to get them apart, do you really think you can get them back together, and have them have the same clearances and tolerances afa friction, that it had originally? I doubt it.
I say do not take apart the motors. How about making four thin aluminum disk shaped mounting adapters that would go into the p3 frame? Same OD as the motors (or probably could use 1" bar stock), 3 countersunk holes to mount the p4 motor to the adaptor, and 4 tapped holes to mount the adaptor to the frame. 1/8" thick should be enough (meaning the motors would sit 1/8" higher. May have to source shorter screws or cut down the OEM P3 screws, because you only have 1/8" to screw the motor/adaptor to the frame. Oh yeah then blue locktite
You would need a milling machine to get the hole positions accurate.