Which is more or "Better" out of those 2? ima guess the 4gb from the ps4 since its DDR5 and not DDR3? im not very good with that kinda stuff.
I would have to say the PS4 will have the real advantage out of the two. The nextbox is rumored to feature a Windows OS built into the console, and about 3gb of RAM of its 8GB will be reserved for it. That would leave about 1GB for VRAM, which leaves about 4GB dedicated to gaming.
Now the next Playstation will have an OS that will only reserve 512MB of RAM of its 4GB. That leaves the rest to gaming. Now, like said before, Sony, according to rumors, will however use DDR5, which should actually be called GDDR5, because it's a graphics based (double data) RAM... meaning it should blow the Nextbox's DDR3 performance.
There are also rumors about the PS4 aka Orbis to sport a 8GB of RAM, but honestly, it's a bit unnecessary for a console (especially if it's gonna have GDDR5), unless you're going to have to have an OS like Windoze. Just remember what these consoles can do with only 256MB of RAM/VRAM. 4GB is plenty enough, and it seems both parties will utilize that much for gaming.
As for the topic in question, I'm pretty confident PD will be able to increase the car count per track. In fact, I'll be surprised if they didn't. If not, I'm pretty sure it's not because they can't, rather they decide to do some serious prioritizing elsewhere into the lighting engine and physics.