Four differnet types might be excessive, but I certainly think there's a case for a limited number of ASW/ASuW types (P-8 perhaps) to be augmented by a larger number of a cheaper to procure and operate maritime surveillance type (Saab MSA, L-3 Q400, or Boeing MSA spring to mind).
Of course, in these days of reduced aircraft types I accept that idea might not fly, so to speak.