I have a question about this before I buy. So my question is, if I import my map into UE4, will I be able to edit the textures on individual sides of a model like in Hammer?
Not at the moment, I'm afraid. If it's a single brush, you can export the fbx to your favourite editing program and re-import. If it's multiple brushes, you should be able to tell HammUEr to only reimport those after you've modified them in Hammer.
(Epic are working on some kind of in-editor mesh modification thing, tho, although I've only seen it demo'd with VR?)
Ok. However, If I do use textures on different faces will it also be transferred over to UE4?