I think that GT7 when fully patched and updated towards the next couple of years will have as such single player content as GT4, or even better.... This game is basically still incomplete and rushed, which is a shame since most people want to enjoy it fully at launch time, but what gives me hope is that it will get much bigger with constant updates, so we have to be patient, sadly.
I would prefer to have a full career like GT4 with 400-600 races and many modes at launch, but since I know that the game will grow over the time I have the faith that we will get something like that at GT7 peak, until GT8 comes out.
I doubt GT8 will come before 2026, so they have a lot of time to fix, enlarge and improve GT7.
I am still disappointed by the underwhelming career mode at launch state and even in the current state, it's already getting better but we should have all that stuff already at day 1.... But since todays games deliver at half or even less content of what was originaly planned, adding the rest via DLC or updates, it doesn't surprise me anymore. The times of the complete games at launch period is over from long ago.