Troll armor can't stack up to Iron and beyond, you wouldn't want to upgrade it to rank 4 anyways.

And no, currently the max workbench level is 5.

Originally posted by pamp:

Is there a way to get the bench to level 6?

As of right now, no. We'll see what happens when Hearth and Home comes out here in a bit, but it might be slated for later addition if related to another Biome.