Accessibility

    The controls for Quake IV are not difficult to get the player moving and targeting/shooting.  However, to do more advanced movement and shooting takes time and practice to master.  It is simple to go straight ahead, jump, and shoot.  Strafe-jumping (which allows the player greater speed) on the other hand takes some practice to get down.  Also aiming and hitting enemies with the different guns takes some practice as they all fire differently. 
    With no arms you cannot play the game at all in its current configuration of controls.  You would pretty much have to have the ability to aim and turn and strafe by looking (perhaps with glasses that tracked eye movement), and perhaps some foot pedals to move the player. 
    If playing this game without sight, one would need to play by sound and vibrations.  For instance, the player could move the mouse and when the crosshair was aimed at a player it could vibrate and he could fire.  And when he was in the crosshair of an enemy he could hear a danger sound and could move away accordingly.  Also one would need to know the map, perhaps by studying it and hearing a certain sound when a wall was collided with.  The maps would need to be pretty straight forward for beginners. 
    With one arm a player would definitely need some of the aforementioned equipment.  Perhaps the simplest would be some foot pedals to move and the mouse to aim and fire.