Based on the identification of four-dimensional Möbius transformations
$$ g(x)=(ax+b)(cx+d)^{-1} $$
by the matrix group $\mathrm{PS}_\triangle L(2,\mathbb{H})$ of quaternionic $2\times2$ matrices with Dieudonné determinant equal to $1$, we give an explicit expression for the classification of $g$ in terms of $a$, $b$, $c$ and $d$.