I've got a pretty big f4/f3 nikon system, and a nice compact MF setup; I guess I don't see a lot of overlap between them- I use the f4 when I want to burn a lot of exposures or need something it offers, I use the MF for my premeditated photography when I know I won't need many frames to get what I want. I wouldn't use MF for sports, for example- I've tried, but was unhappy with the success rate vs. processing time for the number of exposures.
As I see it, the F4 has decent tracking AF, TTL flash, long telephoto capability & 37 consistent shots per roll. These are things I need sometimes. I could probably use the f4 for all my MF shots, but they would lack the look I was attempting to achieve, but I simply could not use MF for many of my f4 work.