Yes, it has been confirmed: Halo 3 will include not two, not three, but four-player co-op. In this post on Bungie’s site, the company confirms the inclusion of co-op and details the challenges in making it happen. The mode will be available for up to four players via Xbox Live, as well as System Link with two players on each console.

Apparently, the mode seems to be quite complete as they have been playing it for “quite some time” and having some of their “greatest ever” experiences. Here’s a bit on the gameplay that will be found in co-op mode:

Acting as a team is a fantastic new twist to the Campaign gameplay - something that works as well on foot, in close confines as it does on the battlefield, with large scale vehicle mayhem. Scared of Jackal snipers? Send out a scout to see what dangers lie ahead. Terrified of an open field? Flank your enemies and swipe their rides. The combinations and scenarios are endless - and you’ll be able to enjoy them time and time again in Saved Films of your co-op exploits.

Like every other aspect of Halo 3, this mode is chosen from a Lobby. Simply select Campaign, choose your network (Xbox Live or System Link) and invite your friends to join you, or they can simply join your session once it’s set up and in a lobby (but they cannot join a game once it’s in-progress). They will pop into your lobby instantly, and when you have all the players you need (anywhere from one to four) you can begin the game from the beginning - or from any of your saved checkpoints.

Split-screen co-op is as ever, limited to two players per screen - but they can if they wish join two other friends online or via System Link.

Thankfully, they have decided to add new characters instead of having four Master Chiefs running about. Player one will control Chief himself, with player two joining in as Arbiter. The other two players will don the role of brand new Elite characters, whose biographies have been posted on the site. Interestingly, all characters will have identical abilities, with only their appearance and weapon starts being different.

They’ve also released a nice group screenshots, showing off the four characters posing together. Enjoy: