I just digged into it myself and it is not the same anymore. This time a terrain is divided in four files
I took a flat terrain and then just updated a small patch in height and saved the entire thing. The only two files that got updated where .bullet and .patch. My first guess is that heightdata is saved in the patch file since that file seems to have changed the most.
From what I can tell the first 9 bytes are the string PVersion4. The rest of the file is probably the height data. Although I can't tell if it actually starts straight after or there is something else. I think it does cause the first byte is 00 and I'm working with a mostly flat terrain. It's not until far later in the file the data starts changing.
I tried creating a simple 1 on 1 terrain and it seems starting from offset 00001110 there starts to be other kind of data. I just put one point in the terrain to an insane height, and I see the expected bytes change after the Pversion4 but then all the bytes after 00001110 start to explode as well. So there must be more meaning to them for the heightmap
Again I would be very very very grateful if Larian would explain these four files to us. I think I can update my height map importer for D:OS2 but I need some more information first.