I currently have the F2 - love it, but I want to give it to my GF and was thinking of upgrading instead of getting another F2. I'm curious though, since there doesn't seem to be solid specs available anywhere, does anyone know the major differences between the F2, F4, and F75? I noticed when I opened my F2 up at one point that it uses the same board as the F4, but it looks like a section of the board isn't being utilized for the F2.