My secret? Well, hmm... I dunno, but here's the jist of the settings I used:
Springs 13/14
Shocks at 6 all around
Ride height, dumped in the weeds, (55/55)
Camber 2.5/1
Toe 0/0
Sways 6/6
Tranny is gear trick, with a 4.560 final, a maxed out first and second gear, then the rest adjusted to blend in, thus I'm using 2nd in the tighter stuff, and going up to sixth and almost redlining it coming out of the tunnel.
Limited slip is at 5/10/5, but I was considering using something like 5/12/5 just for a little more traction in some corners, but I didn't get that far, and it might be worse coming out of turns anyway due to it being harder to rotate. I also had a stiffened car, a non-stiffened car is something that I'd like to experiment with, but I don't think I'll be seeing GT4 again before this week's end.
I was in sunriver for a few days, that's why I hadn't posted anything earlier
Oh, and no aids, and full downforce.
Good luck!