Genuinely though, how much of that is related to the entire team's offense being comically bad?
Lundestrom (yes even the guy who already can't do offense is snakebit) and Leason are shooting pretty far below their career percentages this season. Harkins is also pretty low, though he's roughly in line with his career percentage. (Johnston has the best percentage of the four but I don't think his one goal on nine shots is a big argument in his favor.) Other than him though, by fourth line standards they are generating some stuff, just not finishing. It's one thing if it's just them, but this has been a common complaint about a lot more than the fourth line this season, under a coach who explicitly thinks offense is a thing that "will just happen." So there's that.
There's also McGinn, the one guy who was shooting about on pace with where you'd expect him to be. His finishing is presumably not relevant going forward, but his goals probably are relevant to the playmaking ability of his linemates. If you include him, the fourth liners have provided 16 of the team's 162 goals. (If anyone wants to be real depressed, the power play has provided 21.) In any case, getting nearly 10% of your goals from your fourth liners doesn't sound that unreasonable to me? Are there expected splits on that?
Like I'm not saying there are no improvements to be made on the fourth line, obviously there are. (Even now they could probably be improved just by keeping Colangelo up and putting Fabbri at 4LW when/if he comes back, or bringing up Nesterenko if the Gulls' playoff run fizzles. I like Harkins' motor but there's probably not much more production to get out of him. And firing Johnston into the sun would be a general improvement.) But I do think they're getting an outsized amount of grief recently, and likely suffering from the same coaching issues as everyone else.