4

"Four is the door."

Four is often seen as a cycle.

Traits of 4

 * Composite number
 * Square number
 * Even number

Examples of 4

 * Four bases make DNA and RNA
 * Four chambers of the heart
 * Four blood groups: O, B, A and AB
 * Four main brain waves: alpha, beta, delta, theta
 * Quadrupeds
 * Four hits in baseball: singles, doubles, triples, home runs
 * Four Horsemen of the Apocalypse
 * The Four Gospels
 * The Four Noble Truths in Buddhism
 * The Four Rightly-Guided Caliphs
 * Four elements: earth, air, fire and water
 * Four directions: north, south east and west
 * Four seasons: spring, summer, fall and winter
 * Four suits of playing cards: hearts, clubs, diamonds and spades
 * Four states of matter: solid, liquid, gas and plasma
 * Four parts of structure: points, lines, area and volume
 * <p style="margin-bottom: 0in">Expressions: “Square deal”, “Square meal”, “Fair and square”, “Back to square one”, “Planted squarely on the ground.”
 * <p style="margin-bottom: 0in">Four corners of the world
 * <p style="margin-bottom: 0in">Origami always starts with a square
 * <p style="margin-bottom: 0in">The Tetragrammaton