If you have not bought either I would strongly suggest EZDrummer. It is by far the better of the two. I have been using both a long time now. Well, “using” GA4 is perhaps not the right term. “Tried to use and given up” is the right term. Entering the Toontrack world of EZDrummer is like playing in premier league compared to some obscure club just founded by fat 40 year olds trying to play football.
I think GA4 is such a bad product that the best advice to Steinberg is to pull it from the market and come back next year with something new (in the direction of GA2/3).