I have not heard the Qs4's, but I think most will tell you the Qs8's will give you better performance overall.
The Qs8's 400W max amp power, versus 175W on the Qs4's.
The Qs8's have a little better low end freq response, most likely to their dual 5.25" woofers versus the 4" woofers on the Qs4's.
Don't get me wrong, the Qs4's are mighty fine and have a proven track record, the Qs8's are just their bigger brother. Maybe if you order everything from the Factory Outlet, that will help you make your decision.
