If you NEVER use more than 4 gigs, then use 4 gigs of 8500--assuming it is in fact running at that speed and your motherboard supports it.
If you SOMETIMES use more than 4 gigs, then use all 8 regardless of their rated speed. You would have to monitor your usage to determine if that was true.
More is better, assuming you use it at least occasionally.
As far as I know, if you are running RAM of two different speed ratings, Windows will default to the lower speed and all sticks will run at that lower speed.