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 --