Fixing the fourth line will not move the needle toward the playoff goal. The fourth line players are good enough for a wildcard team, and for most teams actually.
The problem is the top 6 and it has been for a long time. Improve the top 6, move some of the current 6 to the 3rd line.
Everyone likes Doan. I do too. But he is not a first line player at this point in his career. He would not be top 6 on top 8 team.
Add better players, improve the first line and benefit from the trickle down.