Without knowing a little more about your setup it's going to be hard to suggest anything.

Can your computer handle the 4 accounts?
I, for instance, have 2 REALLY crappy computers running 5 instances of WoW.
Have you tried turning all the graphics down? It might be worth a shot just to test it.

Have you tried setting the maxfps on the slave accounts to a lower setting?
If you use Keyclone, you can set this in the options. If not, I belive you can manually type "/console maxfps 12" in each slave window.

I've recently defrag'd my harddrive with a tool called Jkdefrag and gotten a significant improvement. (use the -a 7 option first then do a normal defrag).

Not sure if any of these will help, but again, without knowing your setup it's going to be hard for anyone to suggest anything.

Good luck!