Four finger swipe not working on Magic Trackpad Even though in the preference pane for Trackpad it says "Swipe between full-screen apps" when I'm using the internal trackpad in my laptop, it will switch desktops even if I'm not using full screen apps. However, when I'm using my Magic Trackpad, the 4-finger swipe does not switch desktops.
Solution: Go into settings -> trackpad -> turn off the four finger swipe, and turn it on again.

(Credit to Stiwi for the very efficient Jeopardy Question)