The cube root of \(-64\) can be expressed as:
\[
\sqrt[3]{-64} = -4
\]
This is a real number since cube roots of negative numbers yield real results.
Step 2: Evaluate \( \sqrt[4]{-625} \)
The fourth root of \(-625\) is not a real number. In the complex number system, it can be expressed as:
\[
\sqrt[4]{-625} = \sqrt[4]{625} \cdot \sqrt[4]{-1} = 5 \cdot \left( \frac{\sqrt{2}}{2} + \frac{\sqrt{2}}{2}j \right) = 3.5355 + 3.5355j
\]
However, since we are only interested in real numbers, we conclude that this expression does not yield a real result.
Final Answer
\[
\boxed{\text{(a) } -4, \text{ (b) Not a real number}}
\]