When you think of the fourth line of an NHL team, you think mostly of one thing.
Hits.
Traditionally, the fourth line in the NHL has been the domain of the enforcer, the battering ram, the "energy guy". It's a line, hockey thinking says, that you send out when you want an opposition team physically punished, whether that be with a fight or by being introduced to the plexi hard and often.
This is a line of thinking that's still very much prevalent among NHL fans - it's one that we see all the time when players are signed or traded for - the aforementioned "energy guy" phrase is repeated so often it almost becomes meaningless, and whenever teams are down in a game or working their way through a slump you can guarantee someone will call for the fourth line to "go out and bang some bodies, get a little energy going" or similar.