For a while now, the field of 64 has really been 65 teams, with the 2 worst automatic qualifiers (small conference tournament winners, like Southeast North Dakota State, etc.) playing each other for the right to lose to a #1 seed.
This year, the committee added 3 at-large bids, so there are 68 teams that are a part of the tournament. The "first four" knocks 68 down to 64. Now there are four games before the actual first round begins: the last 4 at-large teams play for an #11 seed (USC vs. VCU) and a #12 seed (UAB vs. Clemson), and the weakest 4 small conference tournament winners play for 2 #16 seeds (Texas-San Antonio vs. Alabama State; North Carolina-Ashevile vs. Arkansas-Little Rock).