noun

• A set of four works of art that are connected, and that can be seen either as a single work or as four individual works. They are commonly found in literature, film, or video games.
• (historical) A group of four dramas, three tragic and one satiric, exhibited together at the festivals of Dionysus at Athens.
• (medicine) A combination of four symptoms.
• (medicine, colloquial) Tetralogy of Fallot.