What did hitler do that caused 3,000-4,000 civilian deaths each day in leningrad?

The German army, operating under Hitler's orders, enforced a military blockade against the city of Leningrad.  Starvation conditions in the city led to the 3,000 to 4,000 deaths per day mentioned in your question.
The siege of Leningrad lasted nearly 900 days and cost millions of lives.  It ranks among the most devastating of military encounters in history.