Licensing. I wouldn't be surprised if they haven't gotten the GT3 at all, or if Polyphony have some sort of exclusivity going on.
Also, with the exception of the Porsche GT2 RS, all of the cars shown existed in previous Forza titles. Maybe that was intentional to keep the Porsche in the spotlight as the only "new" car on display.
You're right, it's a 4 cylinder: