It means you need to find every power of 4 that are in the range of 4-1000 - the power is the number that says how many times you multiply the original number by itself
So 4 to the 1st power is 4 = 4
4 to the 2nd power is 4X4 = 16
4 to the 3rd power is 4X4X4 = 64
And so on... Hope I helped!