I don't know, Loki. I think we've discussed this a lot on GTPlanet. So I wouldn't have the best idea as to why these makes were not included. As I have stated in my earlier days, if GT4 is to celebrate the history of the automobile, Ferrari, Porsche, and Lamborghini would have to seriously be considered. Even without these three makes, you have to admit. GT4 did a damn good job of getting in as many vehicles from as many points around the world as possible. This is the first racing game I know where you could race something from 1886 all the way up to the future. But as far as why these makes weren't included in GT4, I have a similar situation.
For my PC, I have NASCAR Racing (classic), NASCAR Racing 2, and NASCAR Racing 1999 Edition (actually, NR2 and NR1999 are the same game, just that NR1999 has better graphics and more tracks). If any two tracks define NASCAR Racing, two tracks would have to be included- Daytona and Talladega. In the three NASCAR games I have, Talladega was there, but not Daytona. Guess another big track missing from these games? Right! Indianapolis. To me, the deal would come down to maybe what the manufacturers and executives want to see in games. So if someone made a game where you had to save the MGM Grand in Las Vegas from murderous aliens, and if the MGM Grand owners and lawyers wouldn't want it to be used in a game in this context, then it wouldn't happen no matter how many briefcases of money the game maker puts down to make the thing happen.
So my best answer maybe relates to the manufacturers having their products used and utilized as us GT gamers do. Maybe they don't want to see GT gamers come up with drift settings for a 550 Maranello, a Lambo Murcielago flying over a hump at the Nurburgring, or a Porsche getting banged up by GT gamers when GT5 comes out (remember, GT5 is told to have car damage). It may be the execs not making this happen. Does it make the game bad? No. As long as the game can still be complete with lots to do and see, then it's no problem. Sorry this took so long.