Originally Posted by
xliioper
Wrong terminology. You are not booking a single fare when layover is longer than 4 hours for US48. It's a broken fare (a separate fare for each leg, instead of single fare based on O&D) booked on one ticket. If there were IRROPs on first flight, yes, they would reaccomodate you on the second when the two fares are booked on one ticket. Schedule changes would also potentially permit free changes to one or both fares when booked on one ticket. Although it may be cheaper to purchase fares on separate tickets instead of a single one due to end-on-end fare combination restrictions on cheaper fares. If you use Google Flights multi-city search, it will alert you if it would be cheaper to purchase the fares on separate tickets instead of a single one.
Savings are usually not all that significant. The below would save $39 if filghts were booked separately as an L fare + K fare vs. 2 K fares on one ticket. The L fare on DTW-ATL requires A-B-A end-on-end combinations and can't be booked as part of an A-B-C routing.
Disagree on the fare savings not being insignificant. Granted this is just my own anecdotes, but if I want to fly A to D (where B & C are Delta hubs, and A & D are spokes), booking separate ticket A-B + B-D is usually considerably more expensive than single ticket A-B-C-D. When it’s A-B-C-D, both start&end are at spokes. When it’s separate A-B and B-D, I am effectively buying two tickets each with a start or end at a DL hub, and like all airlines DL charges relatively more if you start/end in a hub. Booking A-B-D as one ticket would be ideal, but the 4-hour rule prevents that.
It’s also a giant pain because I have to explain to my travel coordinator why separate tickets were booked and justify why they cost more. If the cost difference is too much, I can’t justify it. Even though A-B-D (or technically A-B, B-D) is usually overall quicker than A-B-C-D, and less risky for misconnects with only one connection needed vs two.