4

4 is the second square number 4 = 2².

4 is the first composite number.
A number is divisible by 4 if the its last two digits are divisible by 4.
The simplest Platonic (regular) solid, the tetrahedron, has 4 vertices and 4 faces. 4 is the only number equal to the number of letters in its normal English expression "four".

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