Given $\tan \theta \sec \theta = 1,$ find
\[\frac{1 + \sin \theta}{1 - \sin \theta} - \frac{1 - \sin \theta}{1 + \sin \theta}.\]
We have that
\begin{align*}
\frac{1 + \sin \theta}{1 - \sin \theta} - \frac{1 - \sin \theta}{1 + \sin \theta} &= \frac{(1 + \sin \theta)^2 - (1 - \sin \theta)^2}{(1 - \sin \theta)(1 + \sin \theta)} \\
&= \frac{4 \sin \theta}{1 - \sin^2 \theta} \\
&= \frac{4 \sin \theta}{\cos^2 \theta} \\
&= 4 \cdot \frac{\sin \theta}{\cos \theta} \cdot \frac{1}{\cos \theta} \\
&= 4 \tan \theta \sec \theta = \boxed{4}.
\end{align*}