What things come in sets of four?

The four horsemen of the apocalypse (death, war, famine & pestilence); the four gospels in the New Testament (Matthew, Mark, Luke & John). A pair of two pairs of shoes, the legs, arms and ears added up of a pair of people, and the heads of four people. A barber shop quartet! Wheels on a car. 4 calling birds seasons the four elements (earth, fire, wind, water)