Disclaimer: The solutions we've shared are just one exciting approach, and there are surely many other wonderful methods out there. We’d love to hear your alternative solutions in the community thread below, so let's keep the creativity flowing!

Let the numbers be $a,b,c,d$.

Modulo $3$, there are $3$ possible values which are $0, 1, 2$.

So, using Pigeon Hole Principle, at least $2$ of the numbers are equal modulo $3$.

Without loss of generality, assume that $a\equiv b \pmod{3}$.

So, $|a-b|\equiv 0\pmod 3$

So, the difference of $2$ numbers is divisible by $3$.