I'm not blaming the loss on the 4th line, they have simply underwhelmed in general. Again, it's not like they're terrible, but once you're past the 10 game mark I think it's fair to start the conversation.
Any line "not scoring" becomes an issue eventually. They are decent/solid defensively, not exceptional. PK has not impressed as mentioned. So yeah you can live with subpar offense (they've literally provide none) if they bring some other positive qualities, but there's basically no physical presence or energy. Being defensively sound is the least they can do at this point.
I have nothing against these guys, I like the players but something is missing so far.