.what new things have they added to the game? Or what makes it different than the others?
I'm talking about its multiplayer
Here we go.
Ordnance In game types where Ordnance is enabled (Team Slayer, Big Team, Regicide, etc.) once you collect enough points from earning medals/kills you can get an ordnance drop. Usually you have three choices. These are usually power weapons, grenades, or temporary boosts to your character such as Overshield or a damage boost. Points will not go towards your next Ordnance until you chose your reward. The Tracker specialization allows you to unlock the ability to reset your three reward options (once per drop) when you receive ordnance.
Loadouts In Halo 4, loadouts return from Reach. However this time they are fully able to be customized. (Game modes such as SWAT and Slayer Pro do not allow this.) When you rank up in Halo 4, you usually receive Spartan Points, which can be used to buy unlocked weapons and mods to put in your loadouts. Over time you can unlock up to 5 loadouts. They consist of six distinct parts.
First, the primary weapon. This includes the Assault Rifle, Battle Rifle, DMR, Covenant Storm Rifle, Covenant Carbine, and the new Forerunner Suppressor and Light Rifle. The cost of these ranges from 1-2 Spartan Points (SP).
You may also choose a secondary weapon. There are three; the classic Magnum, the Plasma Pistol, and the new Forerunner Boltshot. While the Magnum is given to you by default, the other secondary weapons will cost you 2 SP.
Note: The Firepower tactical package will allow you to have two primary weapons instead of a secondary in your loadout.
Next is your grenade type. There is the Frag Grenade, Plasma Grenade, and the Pulse Grenade. Frag is given to you by default and the others cost 2 SP each.
Armor Abilities return from Reach with several new additions. The list of AAs includes Active Camouflage, Thruster Pack, Hologram, Jet Pack, Promethean Vision, Hardlight Shield, Autosentry, and the Regeneration Field. Promethean Vision allows you to see through obstacles, and the Hardlight Shield works very much like a Jackal Shield, only protecting you from the front. While most AAs cost 2 SP, more intricate Armor Abilities will cost you 3 SP.
Note: You will always have sprint.
Here is where things get interesting. You may now add a Tactical Package to your loadout that will give you a very specific boost to your style of gameplay. Most, if not all of them only cost 1 SP. Tactical Packages include spawning you with 3 grenades instead of two (Grenadier), increasing the speed of the recharge for your AA, and even giving you infinite sprint.
Lastly, there are Support Upgrades. While similar to Tactical Packages, they make a smaller impact to your gameplay. Some will increase your weapon swap speed, while others will allow you to view your sensor while scoped in or give you larger ammo capacities. Specializations allow you to unlock more of these.
Specializations Your first specialization will be Spartan-IV, and you will keep it for your first 50 levels in multiplayer. Once you reach level 50, you may choose any of the specializations available to begin. Including Spartan-IV, there are 9 in total. Once you have started a specialization, you may not change it before completing all 10 levels of it. While leveling up a Specialization you will unlock a unique weapon skin (for a specific weapon), visor color, emblem, armor, and Support Upgrade. You do not have to be using a Specialization that unlocks any of these rewards to be able to select them. Once done with a Specialization you repeat until you hit the level cap at 130.
Well, those are the main changes to multiplayer. The rest is up for you to experience.