Grassmann images of tensor product surfaces in R^4

Surfaces in 4-dimensional Euclidean space are the generalization of classical surfaces. They are important for construct geometric model of surfaces taking projections of lower dimensional cases. The Grassmann image of surfaces are also important for theoretical physics. In the present study we consider tensor product surfaces in 4-dimensional Euclidean space  . We give necessary and sufficient conditions for tensor product surfaces whose Grassmann images lay on the product of two spheres.