  A Cayley digraph Cay(G,S) of a finite group $G$ with respect to a subset $S$
of $G$ is said to be a CI-digraph if for every Cayley digraph Cay(G,T)
isomorphic to Cay(G,S), there exists an automorphism $\sigma$ of $G$ such that
$S^\sigma=T$. A finite group $G$ is said to have the $m$-DCI property for some
positive integer $m$ if all $m$-valent Cayley digraphs of $G$ are CI-digraphs,
and is said to be a DCI-group if $G$ has the $m$-DCI property for all $1\leq
m\leq |G|$. Let $\mathrm{Q}_{4n}$ be a generalized quaternion group of order
$4n$ with an integer $n\geq 3$, and let $\mathrm{Q}_{4n}$ have the $m$-DCI
property for some $1 \leq m\leq 2n-1$. It is shown in this paper that $n$ is
odd, and $n$ is not divisible by $p^2$ for any prime $p\leq m-1$. Furthermore,
if $n\geq 3$ is a power of a prime $p$, then $\mathrm{Q}_{4n}$ has the $m$-DCI
property if and only if $p$ is odd, and either $n=p$ or $1\leq m\leq p$.
