"Drivable vehicles aren't the only differences between Quake II and Quake IV. Most of the weapons in Quake IV game are holdovers from Quake II (pistol, machine gun, rocket launcher, railgun, etc) but Quake IV also has some upgrades that your friendly marines give you for those weapons. Speaking of which, those marines are also a new element in Quake IV with some decent AI that will be able to help you out in certain situations."