These are the fixtures where 1, 2, 3, 4 represent the teams.
1 plays 2, 1 plays 3, 1 plays 4, 2 plays 3, 2 plays 4, 3 plays 4. In these games each team plays all the other teams just once. There are 6 games here, so if each team plays the other 3 times then the total number of games = 3×6 = 18 games.