Depends. A lot of the "GT4 assets" have been upgraded over the years, and are now more akin to mid level PS3 assets. Like this:
If they continue to upgrade the older assets, i don't really have any problem with them being included. The model on the right, combined with a better render technique and improved lighting, would look perfectly acceptable on the PS4.
It will be interesting to see how the premium PS3 assets look on the PS4. PD put a lot of work into future proofing their premium GT6 assets, including added tech to the engine that we didn't even see in Forza 5 (and was functionally useless for a PS3 game :p).