In an interview with the Official Xbox Magazine, Street Fighter IV-producer Yoshinori Ono says that Super Street Fighter IV will be the final game based around number four in the series. He is very aware of how Capcom milked especially Street Fighter II for all it was worth, and by doing so reduced the overall value of the series.
There might be a patch for Super Street Fighter IV, if it needs balancing at some point, but there won't be a brand new game. We can always hope that this means Capcom is busy brainstorming for part V...