4

Definition
$4$ (Four) is:


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


 * 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.