If we were basing the Top 5 on resumes as of right now, this would be my vote if I had a say in a ballot:
1. Idaho (3-1)
2. South Dakota State (3-1)
3. North Dakota State (3-1)
4. Montana State (4-0)
5. South Dakota (2-1)
One could make a strong case for Idaho to be voted #1 in the poll right now. Two strong performances against #3 Oregon and Wyoming, a dominating win over ranked Albany, and a ranked road win over Abilene Christian.
South Dakota State could also be made a case for retaining the #1 spot. With them being the 2x defending national champs and not having lost to an FCS team in several years, I would say they should keep that #1 spot until they are defeated by an FCS program. However, they have shown to have some early-season struggles that they are working out. However, they still look good and definitely are top 2.
NDSU again, by default, is #3 due to being NDSU. They just find ways to win.
And for Montana State... of the four top 4 teams, they have the weakest resume by far. A 4-0 start is not all that surprising, and for anyone who just looks at team records, they're likely saying, "Montana State is a power program, they're 4-0, they must be really good this year". As others have said, Montana State really hasn't had any real challenge outside of FBS New Mexico... and New Mexico would be a middle-of-the-pack Big Sky program. That Idaho/Montana State top 5 matchup in mid October is going to be a hell of a game.