4. Let $a, b, c$ and $d$ be real numbers greater than 1. Calculate the value of the expression

$$
a^{\left(\log _{b} c\right)-1} b^{\left(\log _{c} d\right)-1} c^{\left(\log _{d} a\right)-1} d^{\left(\log _{a} b\right)-1}
$$

if you know that $\log _{b} a \cdot \log _{d} c=1$