Since the third game in the series was Wario Land: Super Mario Land 3 , some fans consider Wario Land 4 for the Game Boy Advance to be the spiritual fourth entry.
A notorious bootleg game exists for the Game Boy, which is actually a hack of a game called Crayon Shin-chan 4 with Mario sprites swapped in. Download Super Mario Land 4
Below is an article exploring the mystery and the history of this "missing" sequel. Since the third game in the series was