Myst IV: Revelation was always the fourth game of the Myst
storyline series. URU: Ages Beyond Myst was a game outside of the
Myst storyline (taking place in the present time) that acted as a
segue into the URU MMORPG. That URU was released after Myst III is
an accident of development schedules. Myst IV: Revelation is the
true fourth game in the series.