No it hasn't, hybrids are done by the consumer, not PD, PD have nothing to do with them, a Hybrid is created by cracking the code and screwing around with it. If you've heard that a Hybrid is confirmed in GT4, it will be the Toyota Prius hybrid, which is a new duel fuel hybrid car.
I was playing GT2 the other day and was stunned that the Toyota Prius was in the game! the description was pretty funny and optimistic, but they got the hp almost right, something like 57 hp.
They managed to make iLink mode foolproof enough, so we'll see how they fare with iLink mode. I'm not too worried though that they will manage to make it cheater free ... it shouldn't be too hard to check the car specs, as I could do that with the replay manager code easily too .