Homogeneous hyper-Hermitian metrics which are conformally hyper-K\"ahler

Let $g$ be a hyper-Hermitian metric on a simply connected hypercomplex four-manifold $M$. We show that when the isometry group $I(M,g)$ contains a subgroup acting simply transitively on $M$ by hypercomplex isometries then the metric $g$ is conformal to a hyper-K\"ahler metric. We describe explicitely the corresponding hyper-K\"ahler metrics and it follows that, in four dimensions, these are the only hyper-K\"ahler metrics containing a homogeneous metric in its conformal class.