You misunderstood. While the training manager screen operates as you said, four player pops ARE possible.
1 position can train 2 players , starter and backup
2 position = 2 starters , 2 backups , or 3 players, or 2
3 position = 6, 5, 4, 3 .. though I think 6 pops is rare. Wow so busy for the trainer.
5 of course can get four pops. An incredible trainer could get ten but this is never seen.