I was looking for one specific four character hanja expression (as you do…) when I came across these two:
상산구어 (上山求魚): Seeking a fish on a mountain.
Related: 연목구어 (緣木求魚) : Seeking a fish from a tree
They are pretty self-explanatory: someone is attempting to do the impossible.
Odd as it sounds, I have really come to appreciate four character expressions although it will take a loooong time before I will be able to use them naturally. Just had to share these two 😉