Computes the probability density function of the four-parameter JSB distibution given by
$$
f\bigl(x\big|\Theta\bigr) = \frac {\delta \lambda}{\sqrt{2\pi}(x-\xi)(\lambda+\xi-x)}\exp\Biggl\{-\frac{1}{2}\Bigg[\gamma+\delta\log \biggl(\frac{x-\xi}{\lambda+\xi-x}\biggr) \Bigg]^2\Biggr\},
$$
where \(\xi<x<\lambda+\xi\), \(\Theta=(\delta,\gamma,\lambda,\xi)^T\) with \(\delta, \lambda> 0\), \(-\infty<\gamma<\infty\), and \(-\infty<\xi<\infty\).