T4 or T-4 is used for various purposes as a designation, abbreviation or identifier:

• T4 is the IATA code for Hellas Jet airline
• In endocrinology, see Thyroxine.
• In topology, see Normal space.
• The T4 phage is a bacteriophage.
• The T-4 Euthanasia Program was established by Nazi Germany in order to maintain the supposed purity (eugenics) of the so-called Aryan race by systematically killing children and adults born with physical deformities or suffering from mental illness.
• An aircraft, the Kawasaki T-4
• T4 (Channel 4) is the weekend morning teen-aimed slot on Channel 4 (UK).
• In Canadian tax law, a T4 is a renumeration of income tax and other deductions taken by the government over the period of one fiscal year, and is used in filing an Income Tax Return.
• T4 spinal nerve
• Fourth thoracic vertebra of the vertebral column, in human anatomy
• T4 is the short name for Marc Miller's Traveller Published by Imperium Games, Inc.
• The T-4 Atomic Demolition Munition was a small tactical nuclear bomb.
• The Terminal 4 of Madrid Barajas International Airport.

T4