Saints Row: The Third may not be out yet, but THQ has a pretty good idea of where the next one could go. The ever-outspoken Danny Bilson teased a potential sequel, saying that "I already know what Saints Row 4 is about, and it is wilder than this one."
"It's a good example of how you take what you have and say, 'What can happen in the next?'"
Based on the context of the X-Play interview, it doesn't appear Bilson is confirming work on a sequel, but rather explaining his confidence on having a franchise road map. Still, it will be hard to top a game that has you fighting zombies with dildos.
Before you get too excited over Saints Row 4, don't forget that The Third has yet to come out. And after its release, THQ will be prepping an onslaught of DLC.