Don't use the specular option in UE4 for your average specular textures, it's not going to work like how you expect it to, and it's mostly left in for legacy purposes.
activate the image bounds put the cursor to 0,0 drag the uvs to the bottom-left corner use the cursor as manipulator and scale until the uv touch the bounds
Some of the characters of EXILED: From left to right: Ahdiara, Corvent, Lost Soul Demon. Thanks to Alberto Martinez for his contribution (Ahdiara), http://amr3d.carbonmade.com/