In the early attacks around Ypres, in 1916, the crews didn't have anything apart from their service caps. But by the time of Cambrai in November 1917, the crews had been issued with a thick leather helmet ( think early bump hat). They were also issued with a leather covered steel mask with slotted eye holes. And hanging down below nose level, was a chain mail skirt to protect the lower face and neck. In fact the armour on the Mk IV was actually an improvement over the MkI's.