19. BYU (4-0)
SP+ and FPI rankings: 42nd and 37th

SP+ strength of schedule to date: 47th

What the Cougars did in Week 4: def. Kansas State 38-9

Last week's ranking: 31st

Why they're ranked here: Over the past three seasons, BYU's arc hasn't been rise-and-fall so much as just ... fall.

2020: fourth in SP+ (ninth on offense, fourth on defense)

2021: 26th in SP+ (28th offense, 32nd defense)

2022: 66th in SP+ (47th offense, 76th defense)

2023: 76th in SP+ (75th offense, 94th defense)

That's three straight years of regression on both sides of the ball. The offense isn't trustworthy this year, but the defense has rebounded with vengeance, especially against the pass. The Cougars held Kansas State to 5.1 yards per play and forced a trio of turnovers to turn a tight game into a blowout win. A couple more wins like that, and they'll reach "Big 12 contender" status.

Next test: Week 5 at Baylor