It could be done, but it would take an awful lot of work.
Halo developer 343 Industries has said it would be a lot of work to add 4 player co-op to the first two entries in the series.
In an update on Halo Waypoint, community support and engagement coordinator Tyler “Postums” Davis and senior software engineer Sean "Scoops" Cooper answered a question from the community about whether it was possible to bring four-player co-op to the campaigns of Halo 1 and 2 in The Master Chief Collection. The short answer? "Sadly, no." The long answer? "Well, yeah, we could but it would be *a load* of work," apparently.
"I absolutely love this idea, but in the grand scope of body of work in the teams backlog of need to do and want to do, this one is further down the list," Davis wrote.
"Cracking open the old games to make this an official feature is about as big of a 10 as possible on the complexity scale. Now, that’s not saying it can’t be done, but in terms of effort to impact ratio, I think it’s not as far up there as updating the old co-op net code, or bringing long lost content to the games, or even making other global upgrades with collection wide updates. In the future however, I would not be surprised in the least bit if players are able to mod the game content to make this a reality on PC. In the future when major development support for MCC ends up winding down, the modding community will be where MCC lives on. I know this to be true as we have seen many games on PC take on new forms and longer lifetimes for decades now through communities that mod."
Cooper added: "Halo 1 Remastered’s co-op code is very much hard-coded around there only being up to 2 players (or at least the networking parts of it). So to allow for three or four co-op players, that would have to be solved for and debugged first.
"Then you have to solve for the content changes. If I recall, the second player spawns in the cryotube to right of the 1st player. But where would the others? Who are these two new mysterious Spartans? Flash clones of John 117? So more player spawn points would have to be added and verified. Plus other scripting related updates. Which runs the risk of inadvertently impacting existing behavior. Speaking of existing behavior, there’s also the design question of what that means for difficulty. Halo 1 was built with two players in mind, but what happens with more? Is the game now easier? Are there enough resources for all players?
"Halo 2’s existing code is probably more at-the-ready for possibly going to 4 co-op players. However, you still have the same content changes/issues as H1.
"Both Halo 1 and Halo 2 probably also have load time issues to work out if two additional players (and machines) were added to the mix."
Last month, Halo 4 came to the PC version of The Master Chief Collection, making its debut on the platform.