Will there be a 4th book in the mysterious benidict's society series?

Yes, there will be at least 2 more books in the Mysterious Benedict Society Series, Sadly, I don't think that either of them will continue the story of the first three. They are called: The Extraordinary Education of Nicholas Benedict, and Mr. Benedict's Book of Perplexing Puzzles, Elusive Enigmas, and Curious Conundrums. The first is about Mr. Benedict's life as a child; the latter's a collections of puzzles, riddles, and tests, and is not a novel.