Some people would prefer to retain navigation swipe functionality when enabling the new 3-finger drag. This hidden preference will remap navigation swipes from 3 to 4 fingers, displacing the Application Switcher and Exposé gestures instead of navigation swipes.
Type this in terminal and reboot to refresh the trackpad control panel:
defaults write com.apple.systempreferences com.apple.preference.trackpad.3fdrag-4fNavigate -bool YES
[crarko adds: I haven't tested this one.]

