In response to an interview question from Kotaku's Brian Ashcraft, former Kojima Productions assistant producer Ryan Payton let slip that work on Metal Gear Solid 5 is already underway.
There was no follow up question asked of Payton about further MGS5 details, but we do know most of the team behind MGS4 will be involved (sans Payton, obviously).
Said Payton of his recent departure from Kojima Productions: "It'll be hard not being involved in MGS5, but I've got the utmost confidence that Hideo Kojima, Ken Imaizumi, and our amazing team will put together another blockbuster game. I'll be cheering from the sidelines."
Well, now we know why Kojima couldn't pull the trigger in MGS4 and kill off Snake. Sequel money!