This isn't all that surprising -- all you have to do is look at the players that have rotated through that spot thus far this season to understand the decision. DeBoer has tried Stefan Matteau, Ryan Carter, Bobby Butler, Jacob Josefson, Adam Henrique and Dainius Zubrus at first line left wing. Outside of Zubrus, Matteau has gotten the longest look there, but mostly for five or six shifts a game max. In a post I'll (hopefully) get to later today, I'll go deeper into the benefits on the Poni trade, but this is chief among them. I don't know if he'll spent the entire game there, especially if the Devils get off to a poor start again, but I imagine he'll get a pretty long look.
As for the fourth line, I wouldn't be surprised in the least if Barch gets the spot tonight. Going up against a physical Flyers team, you can pretty much bet there will be at least one shift or situation in which the Devils will need their enforcer. Plus, Matteau could probably use a night off to regroup after playing the last few games.
UPDATE: Matteau will indeed be a healthy scratch tonight.