At least for award tickets, DL is VERY strict with the 4-hour rule (not 6-hours). Even if the next-possible connection is more than 4 hours, it is considered a "stop" which is frustrating when you want to use your free "stop" in a more advantageous way. CO and AS are much better in this regard - next possible connection with award tickets is fine, even if >4 hours. I don't have much experience with AA/UA in this regard.