I think the Pro3 even if a graphite version (tub type composite chassis) is a little different than the Pro4 with the woven graphite. The Pro 3 is less likely to glitch for that reason. Composite chassis's seem to glitch a little less than the a non composite chassis like the Pro4.
I know when I ran all my Shumacher touring cars if they had the stock non carbon chassis on I never gltiched but as soon as I went to all carbon fiber chassis setup I would glitch.
That might by why you gltich with the Pro4.