Step 1: Use the definition of cosech.
Given,
\[
\operatorname{cosech}x=\frac{4}{5}
\]
Since
\[
\operatorname{cosech}x=\frac{1}{\sinh x},
\]
we get
\[
\sinh x=\frac{5}{4}
\]
Step 2: Use the hyperbolic identity.
We know that
\[
\cosh^2x-\sinh^2x=1
\]
Substitute
\[
\sinh x=\frac{5}{4}
\]
Then,
\[
\cosh^2x-\left(\frac{5}{4}\right)^2=1
\]
\[
\cosh^2x-\frac{25}{16}=1
\]
\[
\cosh^2x=1+\frac{25}{16}
\]
\[
\cosh^2x=\frac{16+25}{16}
\]
\[
\cosh^2x=\frac{41}{16}
\]
Step 3: Find \(\cosh x\).
Taking positive square root,
\[
\cosh x=\sqrt{\frac{41}{16}}
\]
\[
\cosh x=\frac{\sqrt{41}}{4}
\]
Thus,
\[
\cosh x=\sqrt{\frac{41}{16}}
\]
Step 4: Final conclusion.
Hence,
\[
\boxed{\sqrt{\dfrac{41}{16}}}
\]