Answer
The domain of the given function is the set of all real numbers.
Work Step by Step
The denominator of a rational function cannot be $0$, hence $x^2+4$ should not be equal to $0$.
However, the value of $x^2+4$ will never be equal to zero since the value of $x^2$ is always non-negative for real numbers, making the value of $ x^2+4$ always positive.
Thus, the domain of the given function is the set of all real numbers.