MyAppleGuide found a help screen in Snow Leopard that shows that 4-finger gestures – try it on your trackpad and you’ll see that they’re a little awkward – are coming soon. Not a huge deal, but pretty cool for those with large glass trackpads.
The may hint at a new four finger gesture tab under the trackpad settings that would allow you to customize the action effected. This is similar to the current Exposé and Spaces Preference tab that allows you to use hot corners to call different commands. The file can be found at /System/Library/PreferencePanes/Trackpad.prefPane/Contents/Resources/English.lproj/FourFingerSwipeGesture.nib .