In 1993, John Wells, created a series of previews, music and a map for one level of a fourth game which he tried to convince System 3 to make. System 3 turned him down. Many years later, probably around 1999, the idea was rekindled. Originally set for an early 2004 release, the game was shown at the E3 2003 Expo in Los Angeles, but has since been delayed. Released screenshots showed a game looking very similar to Tecmo’s Ninja Gaiden, also in development at the time (released in early 2004).
The game has also been said to be cancelled at least twice, only to return to development, and the development company (still System 3) does not seem to give much information in regard to the development status. Current rumours surrounding the status of the project indicate it has again been canceled. – [Info from Wikipedia]
[Thanks to Andy Lindfeldt for these screenshots!]