Chronologically Halo 3: ODST is concurrent with Halo 2. Elites do not show in that game because the ones left in New Mombasa were killed by Brutes in preparation for the Great Schism (the conflict between Brutes and Elites seen in Halo 2).
In Halo 4 your enemies are a Covenant remnant faction led by Jul 'Mdama who seeks to finish the extermination of humanity. Jul is of the opinion that humans will hold a very severe grudge against Elites and seek genocidal retribution in the future. Thel 'Vadam (the Arbiter) doesn't enter into the story in Halo 4 but he does still lead a powerful Covenant remnant faction which is a de facto ally to the UNSC.
From Halo: Glasslands:
The Arbiter made sense. There had been a time when the San’Shyuum had made sense, too. Jul wondered if he could actually speak up now, but the words formed and suddenly he could hear his own voice filling the chamber.
“What do you plan to do about the humans?” he asked. “Gods or no gods, they’ll return to their colonies and rebuild them, and they won’t forget what we did to them and how much they loathe us.”
“We’ll consider that if and when it happens.”
“Instead of finishing them off before they regain strength?” There. It was out in the open now. “We should regroup now, while their guard’s down, and exterminate the threat once and for all. Unless you’re too fond of them as pets, that is.”
The chamber was horribly silent now. Jul could suddenly hear the slow shuffling of boots as elders squirmed. He expected Thel ‘Vadam to round on him, but the Arbiter just snapped his jaws together a couple of times in amusement as if there was something he should have told Jul but chose not to.
“The humans say that a fool does the same thing twice and expects things to turn out differently.” ‘Vadam lowered his voice. “It might have escaped your notice that we never managed to defeat them, and we’re in worse shape now than we were a year ago.” Then his expression changed, as if he was steeling himself to break bad news. “We’ve stopped fighting. We need to stop because we can’t rebuild without stability. I plan to reach a peace agreement with the humans, to formalize what has already taken place. Both sides have finally run out of blood to shed, brother.”
As for where the Arbiter is during Halo 4 the likely answer is Sanghelios, the Elite homeworld. He has been trying to rebuild a functional nation there since the Human-Covenant War ended.