I'm a little confused by the wording in this post. If the child is attending college in the Fall 2015 and you want to use the AOTC as soon as possible, then you should withdraw from your 529 the total cost for the Fall 2015 semester minus $4,000. You then pay $4,000 out of pocket. When you do your taxes for 2015, you could then claim the AOTC for year 2015 (2016 was mentioned in the original post).
If I have anything incorrect, I'm sure someone will correct me.