Recently, I just noticed that my 4 finger trackpad gestures are not working. These gestures include swiping 4 fingers to the left or right to switch pages, and swiping 4 fingers up to get Mission Control. I have not done anything differently and have been using these features a lot lately. Also, I have checked in Settings to make sure the box is checked for these features. How do I fix this? I am running Mountain Lion on an early 2011 13" Macbook Pro.