/Tuned or Concept by GT cars were never in GT4, and the ones in GTp are specific models only because the tuning shop was not featured in the prologue, meaning the only way to get a tuned version of your car was through Concept by GT. Oh and quick tune doesnt count because it is basically car setting like in every GT but with the option of upping the bhp.
I thought there was a section in GT4 where you could buy "tuned" cars like the HPA Golf, I sold my PS2 and games last year so can't check.
About the tuned cars. I doubt there will be many of them though. Because think about it there's not going to be a tuned version of every single model. Maybe only 15 or 20. So it wont be that much of a big deal.
But something else I've been thinking.. Maybe you can upgrade your car to the tuned version. Like that thing they had in GT2 where you can convert it to the race car and all that. So instead of having it as a separate car, it's an upgrade after you've purchased it. I might be wrong but it's an interesting thought in my opinion.
Edit: Or they'll have it both ways. Some tuned versions and for the rest of the cars it's offered an upgrade.
Sounds feasible and would be useful to differentiate between completely stock unmodifed cars and cars the player has upgraded.