Four-finger swipe between desktops does not work with wireless magic trackpad

That worked. The solution is on page 1. Toggle to 3 fingers then back to 4 fingers on the swipe between full screen apps option.

I upgraded to Mountain Lion just today (10.8.2), and after toggling on all options ("three finger drag" was somehow turned off again), they stay on and work, even after a reboot.

This was driving me nuts - many thanks for posting the work-around to a problem Apple haven't bothered to fix in over a year already.

I also reported this as a bug on apple.com/feedback (I assume many of you have already done this).

Perfect! Thanks, Richard.