You should never over lap channels that causes co-channel interference. If you have to then as stated above the AP's that are farthest physically apart should use the same channel.
Also, I would turn off 2.4GHz unless you NEED IT and even then you probably don't NEED IT on all 4 AP's. Try just on 2 AP's to reduce interference and see what happens. Many cases in our offices I have either completely turned off 2.4 or I have it on only 1 AP and absolutely no complaints about wifi since I did that.