The scalar curvature and the biorthogonal curvature: A pinching problem

The famous pinching problem says that on a compact simply connected $n$-manifold if its sectional curvature satisfies $K_{min}>(1\mathrm{/}4)K_{max}>0$, then the manifold is homeomorphic to the sphere. In [8, problem 12], S. T. Yau proposed the following problem: If we replace $K_{max}$ by the scalar curvature, can we deduce similar pinching theorems? In our present note we give an answer to this question in dimension $n=4.$}