An open problem is proposed of characterizing the linear maps on tensor product operator algebras that preserve the numerical range of simple tensor product operators.A characterization of linear maps on M4=M2(C)←M2(C) which preserves the numerical range of simple products of the form A←B
where A
B∈M2(C)
is given and an outline of the proof is presented.Ther are examples to show that the same characterization is not valid for maps on higher order.