Originally Posted by
xliioper
DL does not have any prohibition on connections over 4 hours. AA, DL, and UA all have the same 4 hour limit on US48 domestic fares (before layover becomes a stopover). Anything showing longer than that on their websites will simply be broken fare constructions (separate fares on each segment). The fact that AA/UA may price out broken fares more readily on one-way/roundtrip searches doesn't mean that DL prohibits such constructions. You can readily price out US48 connections longer than 4 hours on delta.com using multi-city search if that is your desire. While pricing out on separate fares is often more expensive than on a single O&D fare component, this isn't always the case.
WN is an exception compared to UA/AA/DL and allows layovers up to 8 hours on a single fare component currently.
AA US48 fare rule example --
UA fares rule example --
WN fare rule example --
I book through fares on AA all the time with iver 4 hrs connection. (Overnight usually, so it may be instead they are going by the rule of 1st flight next day when no onward left same day). Booking as a broken fare at an AA hub would be much more expensive.
If I remember AA tried to go back to the 4hr rule, started and gave up. (Leaving some routes with numerous options with 4hr, but most other routes not changed) If they truly went back on all/most, it's not obvious on routes I fly them on.