Okay I think the arbiter should be in halo 4, so I made a sort of arbiter trailer. Well didn't make one, but designed in my head how it should go.
So the music playing in it will be Mad World.. Basically it will show the arbiter, in his old arbiter slowly through a burning sanghelli city. It shows burning buildings, dead elites and humans on the street. He comes to a cross section of two streets, when he sees the dead body of the Shipmaster. He kneels down, and gives a salute of respect to his dead friend. When he gets up, he looks around and sees nothing. Then hundreds of soldiers all wearing forerunner combat skin come out of active camo, and arbiter is surrounded. They aim their weapons, he draws his energy sword, and the screen goes black, and the halo 4 Icon appears.
A few seconds pass, and it shows arbiter standing in a lone hallway in some unknown building. A shadowy figure approaches him, and we hear him say in gravemind's voice "Tell me Thel valdamee of Sanghelios, do you realy believe you can prevent the evil of my legacy, without his help?" The figure steps into the open, to reveal it's self to be the precursor, and the screen goes white.