Four is the most nearly universal of the sacred numbers, and the reason is not mysterious. Standing anywhere on the earth and facing the rising sun gives four directions, and a very large number of traditions have organised space, colour, season and spirit around that fact independently of one another.
From the directions the pattern spreads. Greek natural philosophy has four elements, and the medicine built on it has four humours to match. Chinese cosmology has four seasons, four emblematic creatures and four quarters set around a fifth position at the centre, which is the arrangement Chinese thought consistently prefers to a bare four. Indigenous North American traditions widely use a four-part division of the horizon with colours attached, and the medicine wheel is the best known form of it.
Indian tradition applies four to the shape of a life and a world rather than to space: four Vedas, four stages of life, four aims of human striving, and four yugas whose lengths stand in the ratio four to three to two to one. Buddhism opens its teaching with four noble truths.
In the Hebrew Bible four rivers go out from Eden and the divine name has four letters, which is why the Greek word for a four-letter thing became its standard designation. Early Christian writers defended a canon of exactly four gospels by appeal to the four zones of the world and the four winds, an argument that reads as arithmetical special pleading now and did not then.