Go through the education section again. Go through the entire education interview until you reach a screen titled "Your Education Expenses Summary".  Click edit next to the student's name. That should take you to a screen “Here’s your Education Summary”. Click edit next to “Education Information”.  You'll reach a screen asking how many times you claimed it before, Change it to something other than 4.

If you have used TT, in the past, TT will have picked up that number from previous year's returns, so be sure you're right. 

If you claimed AOC in the tax year of his first fall term, then you have used up your four credits.

You can get the AOC a TOTAL of four times for the student-----if you have used it four times it does not matter that this is the "4th year" of college.   

4 school years spans 5 tax years ... so if you used the AOC   for the fall semester the freshman year, the fresh/soph the second, the soph/jr year the third and the jr/sr year for the 4th you cannot use it again for the spring SR expenses ... you can use the Lifetime Learning Credit instead.    If you try to use the AOC a fifth time the IRS will reject it.