Boy, I don't say this often, but... what a foolish idea. Now, I've played NFS3 and the first Hot Pursuit, both on my PC. You'd might as well ask if GT4 will have spike strips, police barricades, helicopter pursuit, and whatever else the series has to offer. To me, maybe the Miura, the Diablo GT500 race car, the Diablo Supertrophy car (a joy to own in your virtual garage), maybe the Countach, and the Lamborghini Canto. But, what's the point of having a police car in GT4? They serve only one purpose- to protect and serve. And when racing, why defend the peace while racing against a pack of 5? You'd may as well ask if a Dodge Intrepid taxi cab (like here in Houston) will be in the game. Or even better, an E-150 (from Ford) Ice Cream Truck?
A Diablo police car isn't historically significant or even a good idea for a RACE car. It's a creative idea, but I just don't see that happening from a semi-professional point of view.