Solution
Here is one way to do it:
Since the Transition Mask texture needs the UV to be at 1,1 (not tiled), pick one of the 3 Custom UVs for its TexCoordinate, and Divide by the tiling amount applied to that Custom UV to return it to 1. You can see this in the image below, highlighted in green. This way you can tile 4 (or more) textures using only 3 Custom UVs.
