Aktion T4 (German, pronounced [akˈtsi̯oːn teː fiːɐ]) was a campaign of mass murder by involuntary euthanasia in Nazi Germany...

The name T4 is an abbreviation of Tiergartenstraße 4, a street address...

The killings took place from September 1939 until the end of the war in 1945; from 275,000 to 300,000 people were killed in psychiatric hospitals in Germany and Austria

https://en.wikipedia.org/wiki/Aktion_T4

Aktion T4 is not the same as the alleged concentration camp executions