Well I bought an F4 and used it this year, I never really had a chance to compare to the 350, although my hunting buddy has one still, but bought a ATP relatively soon after starting detecting with the 350. We didn't get to hunt much together until after he got the ATP.
I do like the (standard) DD coil although under certain contitions it will detect bottle caps and call them quarters.
It's nice to have the gb but don't really use it all that much, just me.
My feeling about the F4, although I really do like it and certainly wouldn't recommend against it, is I really wish I would have gotten an F2 instead, and I would have probably skipped the F4 and went with an F5 as my next machine.
I'll spend another season with the F4 and decide if I am going to move up to the F5 or further up to an evan more expensive machine.