Weapons that were used during the events of Halo 4, while listed as "Post-War" in era, likely saw use prior to the end of the Human-Covenant war. The development of a new weapon can last a decade or more sometimes, so it isn't crazy to think that these weapons would be available on a limited basis, especially to members of a Special Operations Detachment of Marines, who mostly will be involved with mission assigned by the Office of Naval Intelligence. That's why I am willing to accept Halo 4 weaponry, at least in terms of the weapons that are simply re-dresses or updates to already existing weaponry; IE, the Asault Rifle, the Pistol, the Battle Rifle etc. etc. etc. The only weapon I didn't include as an option was the Rail Gun from Halo 4, as I see that as a brand new tech of weapon. It hasn't appeared in any previous Halo games, therefore it is probably a Post-War weapon exclusively.

As far as the terminology of Fireteam, indeed it is a term used for a small 4-5 man group. I will probably adjust that, seeing as each squad is roughly three times that in size.