Are you running all 4 instances off the same drive now? Do you have at least 4 separate wow folders to run the instances from?

The SDD disks are quite sexy right now but they are also quite expensive too. I would recommend getting at least 2 and preferably 4 drives(sata) to run the instances off of. If only 2 drives make sure each instance has its own folder. This should help.

I run 3 instances off 2 drives but they all have their own folders. I can run all 3 with graphics set fairly high and get 40-60fps on each instance. My hardware isnt the greatest in the world either.

2ghz athlon 64x2, 3 gigs ram, 2 80gig sata, geforce 8800gts