OsSetTerrainTextures

• a set of texture keys for legacy viewers and map
• a set of texture or pbr material keys for new viewers.
  

so this function allows the change of:

• textures for legacy viewers and map if types == 0 or 2
• textures for new viewers if types == 1 or 2
• PBR materials for new viewers if types == 1
  

The list textureKeys must contain keys or names of 4 textures or 4 pbr materials. They must be of the same type due to viewers restrictions.
This defines the textures or materials the viewer will use to draw the terrain texture
They are ordered from low to high terrain height level, as on viewers World -> Region Details -> Terrain menu tab.
If an entry is a name, then the material or texture must be present on the prim inventory.
If an entry is an empty string, then that level is unchanged.