If you make the 'run' floor wire, remember you will have to have a solid-floored or raised up "house" portion to lock the chickens into when you move the coop. They can't just walk along during the move (as they would in a normal open-bottomed tractor) if there is wire there; you will be unable to move the tractor and/or smoosh their toes.
I'd kind of suggest rethinking, honestly. They won't be able to do much of any scratching thru 2x4 wire (pecking, yes) and it also won't really deter predators that much.
Consider a 12-18" wide flip-down apron that you peg or weight down real well when the tractor has got where it's going. It will conform to small unevenness in the ground and do a lot better job of keeping out predators than a large-mesh wire floor (predators can just get under there and grab the chickens *thru* the mesh) and will enable you to have an open floor so the chickens can scratch.
JMHO, good luck, have fun,
Pat