The 4 hour rule is a much bigger problem in DTW/MSP/SLC compared to ATL, because ATL has so much frequency to many destinations.

For example, it’s not uncommon to see XXX-DTW-LGA-YYY as the only “legal” routing because XXX-DTW arrives >4 hours before the next DTW-YYY flight. So even though DTW may have multiple daily frequencies to both XXX and YYY, if they don’t arrive/depart within 4 hours, ticketing rules make you book another connection just cause.

For Midwest flyers to small markets, this annoyance is very real (and idiotic).