If I train players 1 and 3, then are players 2 and 4 solid enough to be starters/backup for my team or should I sell one of them (player 2 is really weak on both OD and ID)?
Player 2 isn't useful with his defense(s). I'd try selling him. Player 4 can be useful and actually has the chance to be a quality SF, but, again, you'd have to commit lots of time to training OD.