You would export the masks from WM, compile them in Photoshop under 1 texture (RGBA, depending on how many areas you were masking) and then use lerp nodes in your material to mask the different areas in UE4. The landscape materials that UE4 provides can give you a more illustrative idea of what to do. Check out the Elemental demo for example, and I believe there are more landscape specific demos that you could look into that use similar methods.