Halo 3 ended on a bit of a cliffhanger, with Master Chief's fate less than certain. However, it's for exactly this reason that Bungie decided to steer clear of doing a 'proper' Halo 4 and anything to do with Master Chief.
Speaking to the MTV Multiplayer Blog at E3, Brian Jarrard, who's the community director at Bungie stated "Even before the idea to build a game based around Reach came about, a lot of other concepts were explored, up to and including a proper Halo 4, where Master Chief was going to wake up from cryo-sleep and we were going to tell that story."
He went on to confess that "it [the idea] just wasn't as interesting to the team to just pick up where that left off and all the baggage that came with it."
No doubt Microsoft have some plans up their sleeve to bring Master Chief back into the fight though.
Source: MTV Multiplayer Blog