  Let $H(\lambda_4)$ be the Hecke group $\langle x,y\,:\, x^2=y^4=1 \rangle$
and, for a square-free positive integer $n$, consider the subset
$\mathbb{Q}^*(\sqrt{-n})=\left\{(a+\sqrt{-n})/c \, | \, a,b=(a^2+n)/c \in
\mathbb{Z},\, c\in 2\mathbb{Z} \right\}$ of the quadratic imaginary number
field $\mathbb{Q}(\sqrt{-n})$. Following a line of research in the relevant
literature, we study properties of the action of $H(\lambda_4)$ on
$\mathbb{Q}^*(\sqrt{-n})$. In particular, we calculate the number of orbits
arising from this action for every such $n$. Some illustrative examples are
also given.
