The number 4 is the third even number and the first composite number.
4 can be written as a product of primes: 2 · 2.
4 is the square of an even prime number (2).
There are exactly 4 one-digit prime numbers.
Four is sandwiched between two prime numbers: 3 and 5, i.e., 3 < 4 < 5.
4 is the only number that is both the sum and the product of the same two numbers: 4 = 2 × 2 = 2 + 2.
The number 4 (FOUR) is the only number equal to the number of letters in its name in English.
There are four elementary arithmetic operations in mathematics: addition (+), subtraction (−), multiplication (×), and division (÷).
4 can be written using four 4’s: 4 = 4 + (4 – 4) × 4.
The following relation is true: 11 + 22 + 33 + 44 = 1! ⋅ 2! ⋅ 3! ⋅ 4!
A standard deck of cards has four suits of thirteen cards each.
Bridge is a card game played with four players.
There are four principal directions, namely East, West, North, and South.
A plane closed polygon with four sides is called a quadrilateral, also referred to as a tetragon or quadrangle.
It is a Jordan-Polya number, as it can be written as (2!)2.
The phrase the four corners of the earth is used to indicate worldwide.
The ancients classified the physical world into four elements: earth, fire, air, and water.
Four is the highest degree general polynomial equation for which there is a solution in radicals.
Lagrange’s four-square theorem states that every positive integer can be written as the sum of at most four squares.
The four-color theorem states that a planar graph can be coloured using four colours, so that adjacent vertices (or regions) are always different colours.
The smallest non-cyclic group has four elements; it is the Klein four-group.
If we repeatedly add the squares of the digits of a number, the process will eventually lead to either 1 or 4. For example, consider the day: 15-08-1947. Remove the dashes gives us 15081947. Adding the squares of the digits of 15081947 gives: 1 + 25 + 0 + 64 + 1 + 81 + 16 + 49 = 237. Repeating the procedure, we successively obtain: 4 + 9 + 49 = 62 ⟶ 36 + 4 = 40 ⟶ 16 + 0 = 16 ⟶ 1 + 36 = 37 ⟶ 9 + 49 = 58 ⟶ 25 + 64 = 89 ⟶ 64 + 81= 145 ⟶ 1 + 16 + 25 = 42 ⟶ 16 + 4 = 20 ⟶ 4 + 0 = 4⟶ ∙∙∙. As another example, consider the number 190. Repeating the same procedure, we successively obtain: 190 ⟶ 1 + 81 + 0 = 82 ⟶ 64 + 4 = 68 ⟶ 36 + 64 = 100 ⟶ 1 + 0 + 0 ⟶ 1 ⟶ 1 ⟶ ∙∙∙.
