Four is the smallest composite number, its proper divisors being 1 and 2.

A number is divisible by 4 if its last two digits are a multiple of 4.

Also:

• 4 is a square number as $2^2=4$
• 4 is a palindromic number
• 4 is a tetrahedral number
• A quadrilateral has 4 sides
• A tetrahedron has 4 faces and 4 vertices
• A truncated tetrahedron and a tetrahemihexahedron have 4 triangles
• A truncated tetrahedron, an octahemioctahedron, and a cubohemioctahedron have 4 hexagons.