MLE of gamma population variance

Let $x_1=2.1, x_2=4.2, x_3=5.8$ and $x_4=3.9$ be the observed values of a random sample $X_1, X_2, X_3$ and $X_4$ from a population having a probability density function $$f(x; \theta)=\begin{cases} \frac{x}{\theta^2} e^{-\frac{x}{\theta}}, & \text{if }x > 0, \ 0, & \text{otherwise,} \end{cases}$$ where $\theta\in(0, \infty)$ is an unknown parameter. Then, the maximum likelihood estimate of $\mathrm{Var}(X_1)$ equals __________