A kind of virus, more specifically a kind of bacteriophage. T4 bacteriophage, like other bacteriophages, are viruses that infect bacteria.
T4 is one of the cool looking bacteriphages (or "phages"), with a faint resemblance to the LEM, or Lunar Excursion Module, the part of the Apollo Moon missions that actually landed on the moon. The DNA is stored in the geodesic "head", and the bottom part forms a "landing gear". This "landing gear" contacts bacteria, binds it tightly, and injects the T4 DNA into the bacteria. There, the T4 DNA starts it's program of shutting down bacterial functions, and starts making more T4 bacteriphage.
One enzyme encoded by the T4 bacteriophage genome, T4 ligase, is often used in molecular biology.