'Aktion T4' was the German programme to kill mentally ill and handicapped patients, and named after Tiergartenstrasse 4, the address of Nazi Central Office in Berlin (1939/45).
The name may in fact have been coined after World War II, during the Nürnberg trials process, and was used to describe the German 'euthanasia' programme in which doctors killed thousands of people 'deemed incurably sick, by critical medical examination'. The programme officially ran from September 1939 to August 1941, but continued unofficially until the defeat of Germany in May 1945.
During its official existence, 'Aktion T4' resulted in the deaths of 70,273 people, but evidence at the Nürnberg trials indicated that doctors in Germany and Austria continued to kill patients after October 1941 and that the overall total of resulting deaths was in the order of 275,000 people. Research based on files recovered after 1990 gives a figure of at least 200,000 physically or mentally handicapped people killed by medication, starvation, or in the gas chambers of concentration camps.