The scenery is made using 4K textures so, if you don't use that resolution, it won't work at the resolution it was designed with.
It's entirely normal you cannot select 4K from the Addon Manager in P3D V4. That option is only required in FSX, to overcome the FSX limitation that you cannot select 4K texture resolution from the FSX graphic settings but, since you can do it in P3D, is not needed anymore, so it's disabled in the Addon Manager, since you can set if from the normal sim interface.