4 (number)
Four redirects here; for alternate uses, see Four (disambiguation).4 (four) is the natural number following 3 and preceding 5.
| |||
| Cardinal | Four | ||
| Ordinal | Fourth | ||
| Numeral system | quartal | ||
| Factorization | |||
| Roman numeral | IV or IIII | ||
| Binary | 100 | ||
| Hexadecimal | 4 | ||
In mathematics
Four is the smallest composite number, its proper divisors being 1 and 2.
Four is also a highly composite number. The next highly composite number is six.
Four is the second square number, and the smallest Smith number.
In addition, . Continuing the pattern in Knuth's up-arrow notation, , and so on, for any number of up arrows.
A four-sided plane figure is a quadrilateral, sometimes also called a tetragon. A solid figure with four faces is a tetrahedron. The regular tetrahedron is the simplest Platonic solid.
The smallest non-cyclic group has four elements; it is the Klein four-group. Four is also the order of the smallest non-trivial groups that are not simple.
A flat two-dimensional map can always be colored using only four different colors so that all adjacent regions are colored with a different color; three colors are not sufficient. This is the four color theorem.
Lagrange's four-square theorem states that every positive integer can be written as the sum of at most four square numbers. Three are not always sufficient; 7 for instance cannot be written as the sum of three squares.
Four is the number of n-Queens Problem solutions for n = 6.
There are four Arithmetic operations: addition, subtraction, multiplication and division.
The Cartesian coordinate system for a plane has four quadrants.
A tetrahedron, which can also be called a 3-simplex, has four triangular faces and four vertices.
Four is the first positive non-Fibonacci number.
Each natural number divisible by 4 is a difference of squares of two natural numbers, i.e.
Four is an all-Harshad number and a semi-meandric number.
The four-color theorem states that a planar graph (or, equivalently, a map of two-dimensional regions such as countries) can be colored using four colors, so that adjacent vertices (or regions) are always different colors.
There are four fundamental forces of nature: gravity, the strong nuclear force, the weak nuclear force, and the electromagnetic force.
Special relativity and general relativity treats nature as four-dimensional — time and three-dimensional space are treated together and called spacetime.
Alpha particle (helium nucleus) consists of four hadrons.
Valency of carbon (that is basis of life on the Earth) is four. Thanks to
its tetrahedral crystal bond structure diamond (one of the natural allotropes of carbon) is the hardest known naturally occurring material.
Four is the number of nitrogenous base types in DNA and RNA - adenine, guanine, cytosine, thymine.
Many of chordate have four feet, legs or leglike appendages (Tetrapods).
Heart of Mammals consists of four chambers.
A lot of Mammals (Carnivora, Ungulata) use four fingers for movement.
4th finger of human (on the left hand - ring-finger) is moved when little finger moves.
Four wings have butterflies, dragonflies.
People have four blood groups.
People have four canines, four incisor and four wisdom teeth.
Four internal planets in Solar system: Mercury, Venus, Earth, Mars.
Four giant gas planets in Solar system: Jupiter, Saturn, Uranus, Neptune.
Most vehicles have four wheels.
Most furniture has four legs - tables, chairs, etc.
Four horses (quadriga) is the maximal number of horses in one row for carriage.
The four color process (CMYK) is used for printing.
Wide use of rectangles (with four angles and four sides) because they have effective form and capability for close adjacency to each other (houses, rooms, tables, bricks, sheets of paper, screens, film frames).
Four is the only English number that, when spelled out, has the number of letters that it names.
Four seasons: winter, spring, summer and autumn.
Four part of a day: night, morning, day, evening.
Four is the figurative number of corners that the world has: North, South, West, East.
Four (四, formal writing: 肆, pinyin si4) in considered an unlucky number in Chinese and Japanese cultures because it sounds like the word "death" (死 pinyin si3).
In the NATO phonetic alphabet, the digit 4 is called "fower".
In the sport of cricket, a four is a specific type of scoring event, scoring four runs.
The four Greek classical elements are fire, air, water, and earth.
Usages include:
In physics
In chemistry
In biology
In astronomy
In technology
In other fields
Four is: