Thanks. After I asked I actually uninstalled it and then reinstalled under v4. It worked for a while, but then I noticed that all but the "world terrain priority" scenery layer is present in the scenery.cfg and WT refused to start. I was adding some other addons, might be that one of those did something to the scenery.cfg file. I then uninstalled it and tried to reinstall, but then it did not start and gave this error:
Since then I can't open it and even after reinstalling it and rebooting the computer, the problem persists. I tried to manually add the many "world_terrain_xxxx" scenery layers to the scenery.cfg, but it didn't help. Or is there maybe some registry residue after uninstall that causes this problem?
Update: Now I remember that when I first installed WT under v4 the scenery.cfg already did not have all the layers properly added, I think only the "priority" and the first (0001-0002) layers. And it worked that way somehow, for a while...
I did now a fresh install: deleted first every folders of WT and even the registry, then installed again under p3d v4. Still, I get this error on the first start. When I go into the scenery.cfg, there is only one WT layer added, the priority layer, no others. And it's added randomly somewhere in the middle, not as the first, as under p3d v3. I don't know if it should be the first, but still... I have the newest v4.2 version, maybe it's some compatibility issue.
Update: I was thinking but maybe the installer gets confused by the new scenery system of v4. Sceneries are stored and listed not just in the scenery.cfg anymore, but also in separate add-on.xml files, which are in their respective scenery folders. Can it be the problem? Or should I manually add all the numbered WT scenery layers into scenery.cfg?
Update2: I got what is the problem. It is indeed a confusion with the new scenery file system in p3d v4. Now the addons are stored by two parallel systems, either in scenery.cfg or by separate addon-xml files. Now when WT installs it puts it's scenery layer into scenery.cfg, but there will be a discrepancy between area number and layer number, the latter being higher, if there are some addons added to p3d through an addon-xml (so those are not listed by scenery.cfg). When WT sees that the layer number is higher than the amount of areas in scenery.cfg, it gives the error above. After I moved down the WT layers, where the area and layer number is the same, I don't get the error, and WT start as it should. But I guess it won't be ideal, because this was there will be some sceneries with higher priority than WT, which could cause some problems. I'll test it though how it works now.
Ok, now I fixed it by adding all the layers manually, converting all my other addons to scenery.cfg type (so nothing is registered to p3d through add-on.xml) then moved all the WT layers up to have higher priority. Now it starts without the error message and the satellite images load as textures as it should. Looks good again.