Well, is it true, that its 3D (the mapping/navigationwise)? Yes, that mod has some broken (odd) room links. In level 4, for example, both above and below the starting room is the same room, which would be impossible in real life. You can see the effect that can have in the beginning of this video a...

Well, is it true, that its 3D (the mapping/navigationwise)?
There's a video on youtube (with russian music and level 1 to 6), that only slightly explains (not enough to completely understand) whats going on.

Just wanted to ask, how exactly is '4D POP', 4D?

Looking around on google and all I could find was that it has harder levels and an odd way of navigating and that its really 3D and the 5th level is actually a sphere.