The first four is an appeasement of major conferences, basically.
A recent SI/Time piece outlines a story from the late 80s. The basic tenant of the story was the major conferences had gotten together and decided that the bottom two conferences would be eliminated outright from the tournament on selection Sunday. Until the Princeton Georgetown game that was very likely to become policy. Instead what happened was a near upset by Princeton caused that discussion to be taken off the table. The small conferences counter offered a play-in game.
Originally the play in game was just the worst two automatic bid schools. Teams that won their conference tournament and had earned their way in. This preserved their automatic bids but also gave the power conferences an additional at large team in the tournament.
A few years ago, the NCAA decided to expand further. However, they didn't want to minimize the effect of winning small conference tournaments, so instead of having all 4 play in games being automatic bid qualifiers, it was set out that two of the games would be automatic bid schools vying for 16 seeds, and that the other two games would be the final four at-large teams vying for the final at-large seeds.
This year's tournament those seeds were 11 and 12 seeds, however, those seeds can be higher or lower depending on the composition of the rest of the field.