GSX will surely detect P3D4 if is correctly installed. If it's not detecting the sim, it means the registry keys for the simulator are either missing or wrong, which can happen if you "moved" the simulator from a drive to another drive, something you should never do.
I suggest uninstalling and then reinstalling the sim because, in addition to the registry keys the GSX installer checks for, there might be other things that are setup by the installer which might go wrong after having moved the sim manually without a proper reinstall.
