Ys IV: Mask of the Sun - A New Theory is a remake of Ys IV: Mask of the Sun. This version is visually similar to Ys VI: The Ark of Napishtim, having character sprites and 3D environments, unlike the fully 2D graphics of the original release.

The "bumping" combat style of the earlier version has been replaced by the more common system that requires the player to press an attack button to hit the enemy. There are three levels of attack strength for the swords, as well as new elemental weapons and magic spells. The general plot is the same as in the original version, but several events have been altered or added.