According to the official Mega Man website, there is currently no fourth title in the Mega Man Star Force series. The most recent release in this series was Meg Man Star Force 3.
Mega Man Star Force is a similar series to the Mega Man Battle Network series, but completely separate. The series has a total of seven games, with three iterations of the series. This is because each iteration is broken up into multiple games. The first game in the series, Mega Man Star Force, has three different versions: Pegasus, Leo and Dragon. The second game, Mega Man Star Force 2, has two different versions: Zerker x Saurian and Zerker x Ninja. Mega Man Star Force 3 has two different versions: Black Ace and Red Joker. The different versions do not vary in story, but have different in-game abilities. Since the games were released exclusively for the Nintendo DS, players can use the DS wireless feature to add friends who have different versions of the game. This unlocks the unique abilities of the respective games for each player.
The Mega Man Star Force series follows the adventures of Geo Stelar and Omega-Xis. While the third game did not act as a definitive wrap-up to the series, it did conclude all of the story lines of the series. There is currently no news of a Mega Man Star Force 4.