Hi ShadowIce,
Sorry for the delay.
I'll assume your four heightmaps are arranged in a 2x2 grid. If this is not correct, let me know.
1) The first step is to import the southwest heightmap tile as per normal.
2) Use the '
File→Import→Merge heightmap' menu item (
see user guide). Select the southeast tile file. Disable 'stretch to fit', then set the x offset to 4096. Click OK.
3) Use the '
File→Import→Merge heightmap' menu item again, and select the northwest tile. Disable 'stretch to fit', then set the x offset to 0 and the y offset to 4096. Click OK.
4) Use the '
File→Import→Merge heightmap' menu item once more, and select the northeast tile. Disable 'stretch to fit', then set both the x and y offsets to 4096. Click OK.
5) Select the '
Operations->Heightfield->Edge blending->Linear regression of tile scaling' menu item, and set both the 'TileSizeX_px' and 'TileSizeY_px' parameters to 4096. Keep 'BlendEdges' as true, and 'BlendDepth' as 128.
You may still get some seam artifacts in this process that you will have to clean up by manual editing.
As for flattening, try the avalanche heightfield tool with the 'Min. slope' parameter set to 0, and the 'Run length' parameter set to 5.
Please let me know how it goes.
Kind regards,
Aaron.