One fits the 4th line role and the other one doesn't. One also makes less money than the other.
The 4th line role must be to suck, get outshot, be unable to play a break out puck on the boards, and to bleed goals against, while providing zero offense? Ie, at a high level, allow 10% more goals against than goals for.
Is that the 4th line role, because it's about all I've ever seen our 4th line do for the last 6 years. That just means that another line has to pick up that 10% to stay even with another team. It may not hurt you every game, but it adds up.
Here I was thinking a 4th line role would be to outscore the other 3rd/4th line you have to play against for a measly 8min / game at 5v5. I would love to see our #1 AHL line as a 4th line. They would probably contribute.
The traditional definition (checking, tough guys) of "4th line" is similar to the effectiveness of the enforcer in the NHL. They are both being proven to have no correlation towards winning, and to actually hurt your chances of it. Playing effective hockey should be the role of the 4th line. Not role players who are only good at 1 thing, while being terrible at everything else. The "good thing" is not enough to offset how terrible they are at 5v5.