In an interview with CVG 343 Industries' Frank O'Connor talks about what's next for the studio after Halo 4.
"...a lot of people at 343 are already prototyping ideas and software and art for future Halo products", says O'Connor.
He doesn't want to be more specific than that, but given that Halo 4 is the first part of a new trilogy it's reasonable to think that Halo 5 is what's next for 343.
"...work on the game started actually before we launched Halo 4 and continues apace as we cycle staff through the team's vacation breaks. We're excited about the next game", O'Connor concludes.