Unmodified 4+ armour save against hand-to-hand or missile attacks. 4+ save against magic spells or attacks (unmodified).
Nagash only.
Note that although the Armour of Nagash offers a saving throw against the effects of a magic spell, this does not constitute a dispel. It is possible for Nagash to avoid the effects of a spell which devastates accompanying troops, for example. Also note that the Armour of Nagash does not affect Nagash's ability to cast spells.