You can’t apply textures to anything because it’s all [PBR][1] but you can apply multiple materials to the same static mesh.
In your modelling app, assign each poly a face ID - these are referred to as Element Index in UE4 and each static mesh can have multiple Named Slots associated with an element index. An example:
edit: another, much more relevant example:
[1]: https://en.wikipedia.org/wiki/Physically_based_rendering