We don’t really know if the Arbiter will be included in Halo 4, but I’m sure 343 has greatly considered it. Here’s how I think he will be implemented into the story.
Basically, the campaign could be separated into two parts, one following the Chief, and the other following the Arbiter, much like Halo 2. Chief’s adventure will be, well, Chiefy, but the Arbiter’s will stray away from the shield world concept for most of the story.
As most of us know, at the end of Glasslands, Sangheilios, the Elites home-world, is under some sort of attack. I’ve been thinking about who could possibly be capable of assaulting the Elites home at this time, and the only logical explanation I could muster is that the Brutes are launching a planetary invasion of Sangheilios.
Now, this makes sense because the two factions are still technically at war, and the Great Schism apparently lasted up until 2559. The end of Glasslands takes place in 2553, which still leaves six years of fighting to go. The strike on Sangheilios could be the large spark that would reignite the conflict between the Brutes and the Elites, which could span the entirety of the next three Halo games.
As of the end of Glasslands, the Arbiter and his crew have just departed from Earth because they were invited to the Voi Memorial (I’m not sure if that’s the correct name). Remember in Halo 3 when the Shipmaster said to the Arbiter that he wanted to return to their home-world to see if it was safe, and then they set course for it? Well, imagine them returning to their planet being under attack by Brutes.
After this, the story would pick up in any way that would make sense. Perhaps the Shipmaster will engage the Brute fleet in space, or maybe the ship will assist ground-side. Either way, you’ll be playing as the Arbiter trying to repel an invasion of your home planet, and, eventually, you will do so, and during Halo 5 and 6, you’ll be on the offensive. Maybe you’ll be fighting against the Brutes over a newly discovered Halo ring in Halo 5, and the humans will play into it as well, and then in Halo 6 you’ll finish the war off with an invasion of the Brute home-world, Dosiac.
Anyway, the Arbiter’s story will reveal more about the Elites and will provide an insanely awesome side-story throughout the next few games. Maybe Master Chief’s and Arby’s paths will, once again, cross in the future.
Remember, this is just my theory on how the Arbiter will be in the next three games, if included at all.