Probability density function of a random variable $X$ is given below

$$f(x) = \begin{cases} 0.25 & \text{ if } 1 \leq x \leq 5 \\ 0 & \text{ otherwise} \end{cases}$$

$P(X \leq 4)$ is

1. $\dfrac{3}{4} \\$
2. $\dfrac{1}{2} \\$
3. $\dfrac{1}{4} \\$
4. $\dfrac{1}{8}$