4

Definition
$4$ (Four) is:


 * The $2$nd square number, and the first square of a prime number:
 * $4 = 2 \times 2 = 2^2 = 1 + 3$


 * The $2$nd tetrahedral number after $1$:
 * $4 = 1 + 3 = \dfrac {2 \left({2 + 1}\right) \left({2 + 2}\right)} 6$


 * The $4$th Ulam number after $1, 2, 3$:
 * $4 = 1 + 3$


 * The number of faces and vertices of a tetrahedron


 * The number of sides and vertices of a square


 * The number of faces which meet at each vertex of a regular octahedron


 * The number of dimensions in Einstein's space-time


 * The only composite number $n$ such that $n \nmid \left({n - 1}\right)!$: see Divisibility of n-1 Factorial by Composite n‎


 * The smallest $n$ such that $n! + 1$ is square: see Brocard's Problem


 * The only number which equals the number of letters in its name when written in the English language.

Also see

 * Hyperbola can be Drawn through Four Non-Collinear Points
 * Plane Figure with Bilateral Symmetry about Two Lines has 4 Congruent Parts
 * Lagrange's Four Square Theorem
 * Ferrari's Method
 * Four Color Theorem