Complex blending is not necessarily slow. I don't see any problems to define for a mesh a base texture and 4 other textures encoded with a RGBA blendmap.

No one says that the used 4 textures are all the same for every other object. If you have e.g. two meshes for a big wooden surface you are not restricted to use as secondary texture on the one wood panel texture A, B, C & D and on the other one texture A, C, E & F.

You could either duplicate for each object the base material and use the mtlSkin1 ... mtlSkin4 as texture inputs or you use some sort of texture atlassing for the 4 textures (entSkin1 ... entSkin4), in which 4 subtextures are encoded each and you select via the shader skills which of the 4 textures each are used then, given that you implemented tiling for texture atlasses (which -is- possible).