I am wondering if there's anyway I can buy a 4K camera/gimbal and install it on my P3A. Is this possible?

It is possible to install the P3P's 4K camera onto the P3A. It would require for you to install the P3P's firmware after installation and use those updates from there on out. If you still have warranty left on your P3A, however, I would not recommend doing this since the modification would void the remaining warranty. If you do not have any warranty left on it, then have at it.