The Service Enigma M4 was used by the Kriegsmarine (German Navy) in the U-boat war. The machine has 4 wheels and a plugboard (Steckerbrett). The 4th wheel, also called Griechenwalze (Greek wheel) or Zusatzwalze, is a thin one and cannot be exchanged with the other wheels. Furthermore it does not move during encipherment.

In German Naval documentation this machine is called 'Schlssel M' rather than Enigma.
