Kim O'Reilly, CBS Sports

We have had a good idea which teams will fill the top three spots for a little while. Purdue, UConn and Houston have separated themselves from the rest of the bracket to the point that if they each lost twice more, they would still be the top three teams in the bracket.

However, the battle for the fourth No. 1 seed is going to be tight and will be heating up this weekend.

I currently have North Carolina in that spot, but the Tar Heels' hold on it is tenuous.

The team with the best chance to run UNC down is No. 2 seed Tennessee. The Vols have three Quad 1 games left on their regular-season schedule, plus the SEC Tournament.  With the quality of the SEC this season, that could mean a couple of more Quad 1 wins.  Of course, a schedule like that can be a blessing or a curse. The Vols could get buried by it, but I like their chances of navigating it mostly successfully.

Tennessee is at No. 3 seed Alabama this weekend. The Crimson Tide has a chance to push for a No. 2 seed.

Bracketology top seeds

Check out Palm's latest bracket, Bubble Watch and full field of 68 at the Bracketology hub.

Marquette is also in a good conference that gives the Golden Eagles a shot at the fourth No. 1 seed.  They have three Quad 1 games left in the regular season starting Saturday at Creighton. They also face No.1 seed UConn at home. The Big East Tournament should provide some résumé-building chances as well.

Arizona is not so lucky. The Wildcats only have Quad 2 games remaining in the regular season and won't get more than one Quad 1 game in the Pac-12 Tournament. Their chances for upward mobility are more likely to depend on the misfortune of others.

North Carolina is a little better off than Arizona. The Tar Heels have one Quad 1 game left (at Duke) in their traditional season-ending game. Clemson and Wake Forest are also solid potential Quad 1 opponents in the ACC Tournament.

Being the fourth No. 1 seed may come with some prestige, but for all of them except Arizona, being sent to the West Regional far away from your fans may not be a desired outcome. The Wildcats will likely end up in the West Regional as either a No. 1 or a No. 2 seed.