4 Consecutive Integers cannot be Square-Free

Theorem
Let $n, n + 1, n + 2, n + 3$ be four consecutive positive integers.

At least one of these is not square-free.

Proof
Exactly one of $n, n + 1, n + 2, n + 3$ is divisible by $4 = 2^2$.

Thus, by definition, one of these is not square-free.