I’m using information from Glasslands and the new Game Informer.
The Servants of Abiding Truth have won the civil war or have gained a significant upper hand against the Arbiter. They have have a little policy on touching Forerunner tech. If you handle it, destroy it, etc you shall be executed.
Now the UNSC has set up a base inside the Dyson Sphere known as Trevelyan. They take some of the Forerunner tech in there and put it into the Infinity.
The Servants of Abiding Truth find Trevelyan and take it from the UNSC. They don’t go to war with the UNSC but keep their little agreement of “I’ll leave you alone if you leave me alone.”
Now sometime later the Forward Unto Dawn is on a direct course with Trevelyan. Elite sensors pick up the ship and they go out to kill any humans on it. They’re probably like “Oh those damn humans are here?! I thought we told them not to come here. Kill them!”
Master Chief fights off some Elites and Grunts as the Forward Unto Dawn crashes into the Dyson Sphere. There he finds some UNSC bases with new weapons such as the BR85, new Assault Rifle, and other things.
He meets up with Forerunners inside link and fights either them or some other enemy.
On a side note Halo4Follower just talks about anything Halo 4 related, I have created a youtube channel for all canon related things with Halo 4. I even find stuff taht Halo4Follower either hasn’t found or has yet to find. link