Simple idea coming from similar quesions: Power of Two/Three

Add one additional restriction so that num is power of 4 rather than power of 2:

```
(int)(log(num)/log(2))%2 == 0
```

Solution:

```
class Solution {
public:
bool isPowerOfFour(int num) {
if (num <= 0)
return false;
int t = pow(4, (int)(log(INT_MAX)/log(4)));
return (t%num == 0 && (int)(log(num)/log(2))%2 == 0);
}
};
```