In addition, . Continuing the pattern in Knuth's up-arrow notation, , and so on, for any number of up arrows.
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 also:
- The atomic number of beryllium.
- the figurative number of corners that the world has.
- the number of musicians in a quartet.
- The number of strings on a violin, a viola, a cello and a double bass.
- In the expression "fourth estate", referring to journalists.
- The designation of Interstate 4, a freeway in Florida.
- Gang of Four
- Four Daughters
- Four noes and one without
- Four Modernizations
- Four great ancient capitals of China
- Four corner method
- Four-character idiom
- Four cardinal principles
- Four Books
This article is about the number. For the year AD 4, see 4.