4 (number)

Four (4) is a natural number that follows 3 and precedes 5. It is one of the most basic numbers used for counting things. In the Hindu-Arabic numeral system it is written as "4", while the Roman numeral is "IV" (sometimes "IIII" on clock faces).

In Somali it is called "afar". It appears in standard counting: kow, laba, saddex, afar, shan. Its everyday use appears in divisions of time, measurements, and geographic descriptions.

Mathematical properties

Four is an even number and the first composite number greater than 1; its divisors are 1, 2, and 4. It is two squared, or two multiplied by two (2×2 = 4), making it the second perfect square after 1. It is also the second power of 2, placing it in the sequence of powers of two (2², 4, 8, 16).

In geometry, four is the number of sides and vertices of a quadrilateral, which includes the square, rectangle, and trapezoid. A tetrahedron is a three-dimensional solid with four faces and is the simplest of the regular polyhedra. The concept of "four dimensions" refers to the three dimensions of space combined with one dimension of time.

Numeral systems and notation

The modern symbol 4 derives from the Hindu numeral system, transmitted through Arabic and reaching Europe in the Middle Ages. As a written character, four can be written in two common forms: one with an open top and one with a closed top; both are equivalent. In Roman numerals, IV is based on the subtraction principle (5 − 1).

Common uses and custom

Four stands for the cardinal directions (north, south, east, west), some of the seasons in certain climate systems, and the four classical elements in ancient philosophy (earth, water, air, fire). In some East Asian cultures, the number four is considered unlucky due to phonetic associations, and some buildings omit numbering the fourth floor.