How do I uninstall FSDT / Flightbeam for for P3Dv4 and keep FSDT and Flightbeam for FSX and P3Dv3 intact?
In P3D (with V3 you need the 3.4 version), you don't have to do anything, because the sim will auto-discover the sceneries on the first start up, asking to enabled them.
But if you keep using FSX, you'll have to reinstall all of them. Basically, keeping FSX installed, is preventing you from the convenience of not having to reinstall after a reinstall of the simulator, because FSX doesn't support this method.