Quote: "No oversteer...But the under is heavy"
I interpret this as huge understeer, and no oversteer.. am I right?
Try driving on N tires for a while.. S tires will feel like superglue after a while, and all your worries will be gone.
My best tuning tip though, would be to do the opposite of what KiwiBoy mentioned... except the ride height bit

It seems to me like stiff=grip and soft=loose in the world of GT4 suspension.. at least on N/S tires. Also: aids need to be off if you want to tune your car.. or else the ASM/TCS will compensate and blur out the changes you made to your setup. TCS is never necessary on 4WD cars.
I'd go for the stock LSD or a 10/40/10 rear and 5/5/5 front setup, springs at 10F 7R, dampers at 8F 4R, stabs to 3/1 and front camber up to 3+. This is my "default" understeer cure package and it usually shows some promise out of the box.
If nothing else seems to work, maybe you're just entering the corners way too fast. Try my tip with the N tires above.. and go drift more

After a while of that, you'll get a lot less understeer trouble... Another thing when turning 4WDs is keep the throttle input low/controlled. The more you floor it and the more you yank the wheel, the straighter the car will go, but I'll assume you already know that from all the threads around here
IF you meant that the car was oversteering, then I'm with KiwiBoy on the suspension.. but the N tires/drift training still works marvels.
BTW: I think that 750HP is way too much for that car and still expect it to handle OK. Try detuning to about 600 max and soften it up a bit.