I have done over 5 1/2 plates for 11 reps but it was fucking hard on me. Stopped going so heavy after that, did do 20's on 4 plates a side for a while but now i would squat 2 and then on the smith if i squatted. There's no point to doing 4, you work the nervous system too much and not the muscle.
Or maybe as you have trained for longer you can get more out of the muscle doing any kind of weights which is very true. I do 4 now i would feel it so fucking much, where as doing it when i was only training a few years didn't do that, and it's not age.