The surd ( numerator or top line) is defined for all real values of s greater or equal to 4.
When s is less than 4, (s - 4) will be negative and the square root is undefined in real numbers.
The denominator or bottom line is defined for all real values EXCEPT s = 4.
When s = 4 you will be dividing by zero which is undefined.
Taking these together f(s) is defined for all values of s greater than 4.
Therefore the domain for real numbers is s > 4
The domain for complex numbers is s <> 4 because square roots of negatives exist in that set.
