U-D4 was under the command of Lieutenant Commander Helmut Brümmer-Patzig enter service in 
the first U-Flotilla in Kiel allocated. First tests were undertaken to test the snorkel, 
but these were eventually canceled (despite the wishes of the German U-boat-based architect 
Christoph Aschmoneit), since this technique than in the rough North Atlantic can not be used. 
The snorkel has been removed from all three "UD" boats.

U-D4 was used thereafter only as an experimental school and boat, probably because of its 
many defects. From May to July 1941 there was an attempt to boot 3rd U-Flotilla in Kiel, 
then to December 1942 as a school boat 5th U-Flotilla in Kiel. From January 1943 until 
November 1944 as a school boat was the 27th U-Flotilla in Gdynia assigned. In July it was 
used to test submarine refueling from the supply-boat U-490, known as a "Milkcow". 
Then it was converted into a U-tanker and was used to train submarine crews in underwater 
refueling. By the end of 1944 about 220 such exercises were conducted. In November 1944 the 
boat came to the 24th U-Flotilla in Gdynia and in January 1945 to 18th U-Flotilla in Hel.

U-D4 was involved in testing the mat absorbs impulses Asdic codenamed "Alberich". 
In 1943, U-D4 in the group NEK * Sultan subordinated tested S-Mi-Gerät (Such-Mine Gerät). 
The device designed to detect mines and surface ships. Both tested devices for the equipment 
came after twenty ships. At the beginning of 1945 a U-D4 took part in the evacuation of the 
Baltic, carrying from Königsberg to Eckernförde three families with children. 
 
It was withdrawn from service on 19 March 1945 at Kiel and decommissioned on 3 May 1945 in 
Kiel Naval Arsenal and scuttled by it's crew. The wreck was lifted after the war and scrapped.
 
 Footnotes: 
 * NEK (Nachrichtenmittel Erprobungs commando) - experimental unit for radio, radar and 
   hydroacoustic devices.

 Ref: http://de.wikipedia.org/wiki/U_D4_(Kriegsmarine)