Gonna take a swing (and a miss) with the final six cars for Spec IV
Ruf CTR Yellowbird
Chevy Camaro SS '02
Nissan GTR '07 (yeah I know we have a lot of them, but the 20th anniversary of that car is approaching)
Ford Mustang GTD '25
BMW M4 Competition '20
Aston Martin Valkyrie AMR-LMH '25
I don't think the GT500's will show up in spec iv, but instead I believe it should get its own update similar to the Hypercars back in June.
Also as far as the Toyota GR GT and it's GT3 counterpart is concerned, depending on circumstances and by circumstances I mean how long would Forza have exclusivity rights to the car, I think it would come in maybe 2027 and maybe in January of 27, but again this is just me speculating.
And finally regarding the M4 GT3, I personally think it'll come in 27 and I don't think PD would want to bloat Spec IV with a quarter of the cars being BMW, but who knows.