I don't think it's the skybox + patch that is the problem... I think this error is caused if there are too many patch surfaces, and if you have a huge patch it will be subdivided A LOT and create lots of patch surfaces. Anyway, DO NOT MAKE TERRAIN OUT OF PATCHES.
For trisoup terrain:
1. Make a 64 x 64 x 16 gu brush (or 128 x 128 x 16 for lower quality but less work),
2. cut it in half diagonally,
3. make copies of that side by side,
4. go to vertex mode,
5. select some vertices on the surface and the ones directly below them,
6. use numpad +/- to move those up/down, or drag them to the side with mouse