The idea is that the binary representation of power of 4 has all 0 except its highest bit.

If we repeat shifting two bits left from 1, we will get all power of 4.

```
public class Solution {
public boolean isPowerOfFour(int num) {
int mask = 1;
while (mask <= num && mask > 0) {
if (num == mask) return true;
mask <<= 2;
}
return false;
}
}
```