The first four in basketball refers to the play-in games for the NCAA March Madness Tournaments. They are four games played before the round of 64 to determine the final four teams to join the tournament.
The 8 teams playing in the first four are the 4 lowest automatic bid teams and the 4 lowest ranked at-large teams (65, 66, 67, 68). Remember that automatic bid teams are teams that automatically qualified for the tournament by winning their conference championships. 32 of the teams in the tournament receive automatic bids. The lowest automatic bid teams are usually weaker than the lowest at large teams, as they may have been a bad team that had a good run in their conference playoffs.
Two of the games are for 11 seeds and two of the games are for 16 seeds.
The four lowest nationally-ranked at-large teams will compete for two available number 16 seed spots. Teams 65 and 66 will play one game, and teams 67 and 68 will play another, and the winner of each match will earn a spot in the First Round.
The four lowest-ranked automatic-bid teams will typically compete for two available number 11 seed spots. The lowest-ranked automatic-bid team will face off against the second-lowest automatic-bid team, and the third-lowest will compete against the fourth-lowest team. The winner of each game will then advance onto the First Round.
The First Four is played on the Tuesday and Wednesday after Selection Sunday.
From 2001 to 2010, the NCAA tournament used an "Opening Round" model. This model had only one game between two teams, and the winner was awarded one of the 16 seeds.
The reason they needed an opening round was that in 2001 there were 31 automatic bids and 34 at large bids. This means 65 teams got bids for 64 spots, so one team had to be eliminated in an opening round.