How do you challenge a teammate who keeps blaming the back four?

I would deal with it away from the emotional moment after a goal. Speak about the behaviour rather than accusing the teammate of being the problem. Something like, 'When every goal becomes the back four's fault, it makes it harder for us to work out what actually happened' is very different from starting another argument about who made the mistake. You can also bring the conversation back to the whole team. Many goals start with pressure being lost higher up, just as defenders can make errors later in the move. If the pattern continues or the comments become personal, the coach should know because that affects the team environment, not just the defenders. You do not have to accept blame quietly to avoid conflict. The aim is to create a more useful conversation about responsibility, not to win a dressing-room debate.