With your proposed tiers, what about Minnesota in a standalone tier 4?
Indiana already beat PSU on the road and faces NU on the road next Tuesday, so having an empty tier between them would make wins in both games non-upsets.
I don't know the other results between your tier-3 and tier-4 teams when tier 3 is on the road, but it seems like there's a sizable gap there between teams ranked in the 20's / early 30's and teams ranked in the 60's/70's. And that gap seems perfectly Minnesota-sized...