  In \cite{Ghys} it is proved that any morphism from a subgroup of finite index
of $\mathrm{SL}(n,\mathbb{Z})$ to the group of analytic diffeomorphisms of
$\mathbb{S}^2$ has a finite image as soon as $n\geq 5$. The case $n=4$ is also
claimed to follow along the same arguments; in fact this is not straightforward
and this case indeed needs a modification of the argument. In this paper we
recall the strategy for $n\geq 5$ and then focus on the case $n=4$.
