The idea is the same with as follows :

231. Power of Two

326. Power of Three

the only difference is that this problem has to add a condition:x*x == num

```
public class Solution {
public boolean isPowerOfFour(int num) {
int x = (int)Math.sqrt(num);
//1073741824 is 4^15, 4^16 is bigger than int
return(num > 0 && 1073741824 % num == 0 && x*x == num);
}
}
```