The standard form of a hyperbola is $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$, where $a$ is the length of the semi-major axis and $b$ is the length of the semi-minor axis. From the given equation $\frac{x^2}{4} - y^2 = 1$, we can identify $a^2 = 4$ and therefore $a = 2$ (since $a$ is positive by definition). Similarly, we see that $b^2 = -(-1) = 1$ and so, $b = 1$.

The distance from the center of the hyperbola to either of its foci, denoted $c$, is related to $a$ and $b$ by the equation $c^2 = a^2 + b^2$. Substituting what we know, we get $c^2 = 2^2 + 1^2 = 4 + 1 = 5$, and hence $c = \sqrt{5}$.

The eccentricity $e$ of a hyperbola is defined as $e = \frac{c}{a}$. Therefore, for the given hyperbola, the eccentricity is:
$$e = \frac{c}{a} = \frac{\sqrt{5}}{2}.$$
This is because we use the values of $c$ and $a$ that we just found.

By substituting these values into the formula, we arrive at the conclusion that the eccentricity of the given hyperbola is:
$$\boxed{e = \frac{\sqrt{5}}{2}}.$$