It does look somewhat similar to an M4 with the barrel cut in length and the stock missing. Also, posseses a dual drum magazine (judging by the magazine model).
In actuality, the Patriot from both games are references to the Metal Gear Solid series. The Patriot in SFH1 is based off of a pistol used in Metal Gear Solid 3, and the Patriot in SFH2 is The Boss's weapon, also from Metal Gear Solid 3. In MGS lore, the Patriot is a modified Firing Port Weapon (an extremely compact M16 rifle) with a cylindrical magazine and no stock. Within MGS3, the Patriot has infinite ammunition since the magazine is shaped like an infinity sign, but in SFH2, it just has a huge magazine capacity.