  Let $G$ be an edge-colored graph. A heterochromatic cycle of $G$ is one in
which every two edges have different colors. For a vertex $v\in V(G)$, let
$CN(v)$ denote the set of colors which are assigned to the edges incident to
$v$. In this note we prove that $G$ contains a heterochromatic cycle of length
4 if $G$ has $n\geq 60$ vertices and $|CN(u)\cup CN(v)|\geq n-1$ for every pair
of vertices $u$ and $v$ of $G$. This extends a result of Broersma et al. on the
existence of heterochromatic cycles of length 3 or 4.
