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Originally Posted by Nikdunaev

Thank you but still nothing seems to be related in particular to the Mark 4 TDC.
Also I thought I made it clear that I came up with that question after already reading both the Submarine Torpedo Fire Control Manual and the Torpedo Computer Manual for the Mark 3. In fact it was reading these two that made me wonder what the difference between the Mark 3 and Mark 4 is

http://www.usscod.org/tdc.html

The TDC was unique in World War II. It was the computational part of the first submerged integrated fire control system that could track a target and continuously aim torpedoes by setting their gyro angles. The TDC Mark III <USS Pampanito gave the U.S. fleet submarine the ability to fire torpedoes without first estimating a future firing position, changing the ship's course, or steering to that position....Instead of hoping that nothing in the setup changed, a fleet submarine with the TDC could fire at the target when the captain judged the probability of making hits to be optimal. The Mark III computer consisted of two sections, the position keeper and the angle solver. The Mark IV TDC now on USS COD also includes a center section called the receiver.> THIS ALLOWED FOR CONSTANT POSITION UPGRADES The position keeper solves the equations of motion integrated over time. The result is a continuous prediction of where the target is at any instant. Successive measurements of the targets' position are compared to the position keeper predictions and corrections for error are introduced with the hand cranks. The predicted target position becomes more accurate as more measurements make the corrections smaller. It is typical to get an accurate track on the target after about three or four observations under good conditions. The U.S. Navy thus had a system that would point the torpedoes at a target as the fire control problem developed. The TDC Mark IV was the only torpedo targeting system of the time that both solved for the gyro angle and tracked the target in real time. The comparable systems used by both Germany and Japan could compute and set the gyro angle for a fixed time in the future, but did not track the target. Thus the idea of the position keeper, and its reiterative reduction of target position error was unique to the U.S.

hope this helps!