The fourth line has been completely useless in the offensive zone this year. The cannot even maintain a presence in the offensive zone. The best defense is a good offense, IMO. They cannot score when the puck is in their end (unless Vesa Toskala is in your net, but that's another story).
None of Wright, Slater, Thorburn have any offensive skill. All 3 are strong defensively, IMO. Adding a player like Machacek that can create some openings in the offensive zone, while still having Wright and Slater to be rocks in the D zone, will help that line out. I don't expect them to start scoring much, but maybe they can start taking the game to the opponent, rather than simply defending. And Machacek is no slouch defensively. He is not a Wright, but he is far from terrible, he is at least average (better than a Kane, Miettienen, etc).
Just think he would balance the line better and allow the line to start punishing the opponent and wearing them down in the offensive zone rather than being stuck in our own end defending.
I am also a fan of versatile rosters, in Machacek you have a guy who can step into other roles as well and succeed, rather than 4 guys who cannot play anything but 4th line purely defensive roles (Wright, Slater, Thorburn, Peluso).