Sandwiched between the external armor and the internal padding is the most devastating weapon the suit carries: a layer of reactive metal liquid crystal. Woven by molecular tools into a superdense layer, this layer is amorphous, yet it amplifies force, doubling lifting capability and increases reaction time by a factor of five. Such technology did not exist when work began on the Mark I prototypes. — taken from http://halo.wikia.com/wiki/MJOLNIR_Mark_IV#Introduction
From what that says, I'm thinking that its just got reactive crystal layer.