Per the admittedly-unreliable wikipedia...
The T4 Model was the adjunct of the earlier T3 model in nearly every way. The T4 was not an ordinary straight-running torpedo, however; it was the world's first acoustic homing torpedo. It ran at 20 kt (37 km/h) for 7500 m and was introduced in March 1943.
Early in 1933 Germany started development and testing of acoustic homing mechanisms for torpedoes. From the outset of submarine warfare, submariners had dreamed of being able to aim and fire torpedoes without surfacing or using a periscope. The periscope gives away the location of a submarine, and a hull-penetrating periscope greatly weakens a submarine's pressure hull and limits the depths to which it can dive. U-boats also had to come to very shallow depths to use their periscopes, generally about 15 m, leaving them greatly exposed to bombing, depth charging, and even gunfire.
With the introduction of Falke, U-boats could remain more deeply submerged and fire at convoys with nothing to give away their position but the noise of their screws. Rather than aiming with a periscope, the torpedo could be roughly aimed at a sound contact as detected by a U-boat's hydrophones, and the homing mechanism could be trusted to find the target without the need for precise aiming.
Falke worked much like a normal straight-running torpedo for the first 400 m of its run, after which its acoustic sensors became active and searched for a target. The sensitive sound-sensing equipment in Falke required the torpedo be as quiet as possible, hence it ran at only 20 knots (37 km/h); in addition, the firing U-boat was forced to stop its motors. Falke was intended to home on merchant targets, however, so its slow speed was not a great hindrance.
Only known to have been fired in action by three U-boats, U-221, U-603 and U-758, although regarded as successful, resulting in the sinking of several merchants, and its performance rated satisfactory, Falke was rapidly phased out of service. It was replaced by the G7es/T5 "Zaunkönig" (referred to by the Allies as GNAT, for German Navy Acoustic Torpedo), which was faster and better able to home onto the sound of fast moving warships as well as merchant traffic.
Though its period of operational service was brief, Falke was a proof of concept for the acoustic homing torpedo. Its introduction occurred only two months before the U.S. Navy achieved its initial combat success with the Mark 24 FIDO "mine." FIDO was not a mine, but a passive, acoustic-homing torpedo designed for use by long-range patrol aircraft. (It was designated a mine for security reasons.) The initial success with the Mark 24 occurred on 14 May 1943, when a PBY-5 from VP-84 sank U-640 with the new weapon. Most sources indicate that the Germans' first combat success with the Zaunkönig (GNAT) did not occur until September 1943. While the Allies became aware in September 1943 that the Germans had brought GNAT into operational service, it was not until the capture of U-505 in June 1944 that they obtained reliable data on the German homing torpedo.