I have to mention first that I've never driven any RR in FM4 that wasn't on the disc(s). I skipped all DLC after April. No Porsches, no Yellowbird.
You should at the very least go for the Yellowbird - its lovely.
Lifting and diving into the corner is what I have in mind.
OK - yep with most (but not all) RWD/RR cars you should get a good dollop of oversteer and I must confess I do find it in FM4.
Is it 100% right? No not really, but at least its doing it which in comparison to GT5 is a major step in the right direction.
From years of owning a RWD and driving RWDs almost exclusively in my sims, you could say I have an oversteer-oriented driving style. When I enter a corner the way I would in Live for Speed, or Enthusia, or in the real world, FM4 tends to just give me understeer/grip where I expect oversteer. I have to block out my "steer with the throttle" mindset in favor of a "time to manage understeer" mindset. It screws with my racing lines.
We both know LFS and EPR are biased toward oversteer, but tossing mid- or rear-engined cars around in FM4 just isn't terribly punishing. Until you get on the throttle, of course; power oversteer is large and in charge in this game.
I actually find FM4 and LFS to be fairly similar in broad strokes in this area, and while I do still love (and play) EPR it is still a little to tail happy at times.
I've certainly come across lift off-oversteer getting the better of me at times in FM4, again its not always to the right level but at least its in the right direction (no pun intended).
I agree that GT5 sits at the opposite extreme. As I recall, if you so much as
hint at weight transfer in a mid-/rear-engined car you go spinning like a top.
The issue with most RR and MR cars in GT is down to the tyres (again) and that differing tyre widths clearly are not factored in at all. Rear tyres on MR are almost without exception wider than the fronts and RR to an ever larger degree, yet in GT if you have the same compound on the front and back you get the same base level of grip. As a result the rears always cut loose first, even when they shouldn't.
The Yellowbird in GT5 as a result is just an oversteering nightmare, turn-in and it oversteers, brake and it oversteers, accelerate and it oversteers, etc. Its totally and utterly one dimensional and doesn't drive like a RR car at all (yet some people assume beacause its oversteering all the time it must be right).
In FM4 you need to balance the understeer of the car, which is present on corner entry and on throttle application due to the wide rear tyres and rear weight distribution, with its desire to oversteer massively when you lift. As a result you have to very much adapt to the car and it drives with all the characteristics of a 911 (as do the Porsche's). Its not 100% accurate, nor did I ever expect it to be, its is however a very reasonable simulation of how vehicles of this layout should behave (and that's everything GT is not with cars of this nature).