Two numbers are equivalent if they have the same digits, for example $2271$ and $1227$ are equivalent. How many equivalent classes are there between $1000$ & $9999$?
I'm not sure I fully understand how this works, is the answer to this question $\dfrac{9\times 10\times 10\times 10}{4!}$? First calculating the total numbers formed and divindg by $4!$ to get rid of the repetitions.