I'm of the opinion that Gr.4 class should've been scrapped from the game, the GT4 cars are almost road cars, with the tuning system of the game you can apply to any road car the modifications the GT4 cars have IRL, such as sequential transmissions. Some of the road cars with Gr.4 versions in GT7 even have the Gr.4 parts available. Besides, tuning cars is more fun.
Gr.4 is meh, has always been meh in GT Sport, slow and underpowered cars with too much grip. The Alfa 155 is a proper race car and destroys everything in Gr.4.
Gr.3 cars on the other hand are much superior to any road car and you can't make a road car drive like a GT3 car just with tuning. Those cars are worth having in the game, despite also being outdated. They're actually fast and demanding, unlike Gr.4.
IMO the Camaro wasn't used because it was more economical for PD to simply reuse the Corvette. I would have preferred the Camaro, of course.