Well, something had to be left out to bring the price down, especially as they actually added features to the F4. The larger trim knobs and two extra front-panel buttons make the decision even more cruel. Oh, and the stereo return and full-sized XLR outputs are handy too! So that's five improvements that I would actually notice and use.
Just in case it is not obvious, you can assign the trim knobs to be mix level instead, and mix on the front panel. That might be good enough for many applications.