The square root of $-4$ is expressed as a complex number using the imaginary unit $i$, where $i = \sqrt{-1}$.

$\sqrt{-4} = \sqrt{4} \cdot \sqrt{-1} = 2i$

So:

$\sqrt{-4} = 2i$

Would you like further clarification on this concept?

---

Related Questions:

1. What is the square root of $-9$ expressed as a complex number?
2. How do we define the imaginary unit $i$ in mathematics?
3. Can you calculate higher powers of $i$, such as $i^3$ or $i^4$?
4. What is the general form of a complex number?
5. How can complex numbers be visualized on a coordinate plane?

Tip: Remember that when dealing with negative square roots, always represent them using the imaginary unit $i$.