Rigidity of positively curved shrinking Ricci solitons in dimension four

We classify four-dimensional shrinking Ricci solitons satisfying $Sec \geq \frac{1}{24} R$,where$Sec$and$R$ denote the sectional and the scalar curvature, respectively. They are isometric to either ${\mathbb{R}}^4$ (and quotients), ${\mathbb{S}}^4$, ${\mathbb{R}\mathbb{P}}^4$ or ${\mathbb{C}\mathbb{P}}^2$ with their standard metrics.