Simply take the power within the root (I.e. 4) as the numerator of the fraction, and the number of the root (I.e. 7) as the denominator of the fraction.
A way of understand would be using the expression (square root)4. 4 can be changed to 2^2. The power WITHIN the root will then be 2, which will be the numerator of the fraction. The power OF the root itself is 2 as well, which is the denominator of the fraction. This will leave us with 2^(2/2) = 2^1 = 2, which is correct; square root of 4 is 2 indeed.
Hope this helps! :)