What is the probability of rolling a multiple of 4 on a fair-sided number cube?

That's going to depend on how many numbered sides the "cube" has, and how

they're numbered.

If it's literally a six-sided cube, with the faces numbered 1 through 6, then the only

multiple of 4 anywhere on it is 4, and the probability of rolling the 4 is

1/6 or (16 and 2/3) percent.