Swapping out textures is easy with M4. Simply click on the terrain after there’s one of the material instances applied to it, navigate to the “Landscape Material” option in the details panel and click the magnifying glass icon. This will take you to where the landscape’s Material Instance is located, open that up by double clicking and all of the textures that M4 uses in it’s Auto Material can be quickly replaced within the Material Instance.
All of the textures are grouped by what they actually apply to, for instance the “Cliff Coloring & Texturing” group houses the textures applied to the cliff regions, “Foothill Coloring & Texturing” house all of the textures applied to the foothill…and so on.