This happened on United a few months back. Arrived in the a.m. on a flight to ORD and was going home to change clothes and "refresh" my bag for a flight at 6:00 p.m. Went to check-in and they would not allow it for the same 4-hour reason. They said it was for "security" reasons! Sheesh, I told them I can check in on line without an ID and that's more secure than checking in with a live person showing an ID? Then I realized that I may as well be talking to my hand.