Well, PGR4 was really under-advertise and coming out right before Halo 3 was pretty much make it a throw away title.
Racing games are dying genre this gen. Last gen, racing game were view as showpiece for console but now a day it's FPS that reign supreme. I am sure this point didn't fail on Microsoft so they were more keen to just concentrate on one titles and save all their bacon for their FPS masterpiece (halo3 btw). Even Forza 2 didn't get much of the advertisement in the US. The driving games are still going strong in Europe and probably the reason MS still invest in it. (Just as they do for JRPG for Japan region).
In anycase it's sad as PGR4 was a great game and deserve much more support from MS.








