There's really nothing wrong with GT4, it's still a great game. I think though we're seeing the Osborne effect repeat itself. The name comes from a (once) famous computer manufacturer who was selling pretty good computers. They announced even better, faster, cheaper ones just around the corner so everyone stopped buying their current models.
Unfortunately the steep drop in income meant they ran out of money before they could release the new models and the company went under.
Pretty much that's what PD have done with GT4 and GT5P. They've killed off GT4 with the promise of GT5 but now no-one's playing any more (or presumably buying any copies) and they're not even close to finishing GT5.
You'd think with the Osborne effect so well known (and even taught in business school) that PD/Sony could've avoided the shot in the foot they've just given themselves but apparently marketing isn't their strong point