I spent a few days putting together a detailed tutorial on 4 different ways to apply multiple textures to a mesh:
Several of these approaches were new to me when I started using UE4 a few months ago so it was a good exercise to try them all out from scratch.
Mapping multiple textures within the mesh (2 failures and 1 success)
Layering multiple textures within a material
Combining multiple textures into 1 texture that is then used in a material (substance designer approach)
There are a few blender tips when applicable.