Box 2 is less then box 4 what does that mean when I do my taxes?

The amount you owe is less than the fair market value. You do nothing except if you get 1099-C. Check box 5 to see if you are personally responsible for the debt or not.

And if box 5 is checked then what happens

If box 5 is checked then what do I do or what happens ?

And if box 5 is checked then what happens

Box 5 shows that buyer is liable to pay the debt. However, in you case you do not have any debt. I don't think you will get 1099-C.
If you only get 1099-A, it is only for your info. Your U.S. Tax Return: Foreclosure or Repossession of Main Home (http://taxipay.blogspot.com/2008/08/foreclosure-or-repossession-of-main.html)