No oneway, you're just not getting it.
Tiger I, Panther, and the Tiger II all have THREE machine guns. One hull-mounted, one mounted coaxil to the main armament, and one mounted on the Commanders cupola for anti-infintry and AA work.
The hull gun is slaved to the turret. It aims at the same place as the turret, so long as the turret is aiming somwhere within its arc of fire. If you hit the secondary fire button when in the turret or commander's possition, you will fire BOTH the hull gun and the coaxil machine gun if both guns can be brought to bear on your target.
The pintle mounted machine gun (the one on top of your tank) can be fired from possition three. There are multiple mounts for the top gun, but each mount only has a limited arc of fire (23.5 degrees, IIRC). So HTC created possition #4 for German tanks, which allows you to move the gun to a different mount, to fire on a different target.
So in short, possition #4 isn't a weapon. It was simply created as a means of moving your other weapon to fire on a target that would be otherwise out of reach, so to speak.