Science

A tetramer is something formed out of four sub-units.

Astronomy

 * There are four rocky planets in the solar system - Mercury, Venus, Earth, Mars.
 * There are four giant gas planets in the solar system - Jupiter, Saturn, Uranus, Neptune.
 * Four of Jupiter's moons, known as the Galilean moons, are readily visible from Earth.
 * Messier object M4, a magnitude 7.5 globular cluster in the constellation Scorpius.
 * The New General Catalogue object NGC 4, a faint galaxy in the constellation Pisces
 * The Roman numeral IV stands for subgiant in the Yerkes spectral classification scheme.
 * The Roman numeral IV (usually) stands for the fourth-discovered satellite of a planet or minor planet (e.g. Jupiter IV)