If you finish the game on Legendary, there is a secret movie that shows Master Chief waking up in the portion of the ship that was left behind in the unstable Slipstream portal. He then seals himself in a stasis tube, and tells Cortana to wake him up when he is needed. The scene then moves outside the ship and you can see it moving toward some planet that kind of reminds me of Coruscant (not that I'm saying it is). If you want to see it yourself, but don't want to battle through the game on Legendary, then go to YouTube, and search for Halo 3 Legendary Ending. As for the other part of your question, there may be a game using the Master Chief as the main character, but the chance of it being called Halo 4 is very low considering that (as far as I know) all of the Halos are shut down after the destruction of the Ark.