If I run the installer the prompt for the location is grayed out an refers to the already installed version on de drive of P3Dv3.
It's not clear if, with "the already installed version on the drive of P3Dv3", you mean you installed the Addon Manager INSIDE the P3D3 folder, or it's just on the same drive, but on a different folder.
If it's just on the same drive, but on a different folder, just go ahead with installing in P3D4.
If, instead, you selected the P3D3 folder and installed INTO that one, is not a very good idea so, I suggest to uninstall GSX and all any other FSDT products, and reply YES to the question "Do you want to remove the Addon Manager ?"
Then, restart the installer, and you will now free to choose a destination folder again.
The default choice of C:\Program Files (x86)\Addon Manager" is perfectly fine, and the only reason why you don't want to accept it, is if you don't have much space on C: available. In that case, choose any other drive, doesn't matter where you simulators are, and choose its own folder, like D:\ or E:, which will install into D:\Addon Manager or E:\Addon Manager, etc.
Whatever folder you choose, it's best you don't install INTO the sim because, now the installer is allowing you to install outside any simulator and use a shared folder for all the FSDT stuff, installing into the sim is the worse possible choice, because the whole point of installing outside the sim, is that you can uninstall/reinstall the simulators without affecting the FSDT software.