I still don't understand the logic for anyone (not you) saying ZAR sucks as a 4th line option. The only time we saw him there was for a handful of games when he was coming back from injury as a rookie. That's not a big enough sample to to determine anything.
I think Empo summed it up best. On the 4th line, I see next to nothing from him that separates him from Kuhnhackl. No more speed, skill, hitting, offense or anything really. At which point we might as well have Kuhnhackl there and ZAR in WBS getting puck time in an offensive role, to hopefully improve his offensive game. However, if we were putting ZAR with Crosby or Malkin (or perhaps even Brassard), then I think he could be used in a more offensive role, and that we would see more out of him offensively. In that case, sure try it and see how it goes, because I really liked what we were seeing when he was playing with Crosby. But when he was with Sheahan? Meh, extremely bland, to the point that he might as well have been in WBS, because Kuhnhackl could have easily replaced him.