Along with thousands of other people I've been trying to import FSX, and even designated P3d V3, scenery & planes.
Sometimes it works, and sometimes it fails terribly.
Last night I tried to install the Madeira X Evolution (Prepar3d V3 version) scenery that I bought last month.
It installed without a problem. That is, until I tried a flight from LPMA. No ocean, lots of sand dunes.
There's a way to go, yet. But that's ok. Unlike some people on the LM Prepared website, I didn't uninstall V3.
BTW #1 - Photoreal scenery from BlueSkyScenery installs perfectly.
BTW #2 - As I install more and more scenery the P3d V4 startup does slow down just a bit, but nowhere near as slow as V3 was.
A TIP for the BlueSkyScenery - I have all of that in it's own folder outside of all three flight sims, BUT all three sims point to that directory so I don't really "install" any of it. All totaled, I have 151GB of their scenery that is shared between FSX, P3d V3, and P3d V4.