Find the number of $4$ letter words that can be formed using the letters of the word $PISTON$, in which at least one letter is repeated.

To find the number of $4$ letter words having at least one letter repeated, we find the total number of $4$ letter words possible and subtract from that the number of $4$ letter words having no repetition.

$\therefore$ we get the answer as $6^4-{}^6{C}_4$