keyclone will handle this as well, but it's not as elegant as you'd probably like (not a slam against Rob or his product at all, just due to the way the game takes mouse input).

There is an option to toggle mouse-broadcasting, when toggled on your clicks will be sent to each WoW client on the same computer - but each window has to be in focus for that to work, so for blizzard, or Rain of fire, or accepting flight paths or whatever what I do is:

hit toggle mouse broadcast key (broadcast is on now)
click action on first window
watch as action is repeated on each clone
after mouse/focus returns to first window i toggle mouse-broadcast off (otherwise you get some really strange results while trying to mouse look or steer.

there is a slight delay as each window is cycled through, so you won't get the instant all 4 clones casting blizzard at the exact moment, but on my machine it's fast enough to get all 4 blizzards off and hitting the group of mobs / players that I have deemed in need of a good AoE treatment.

In summary:
There are a few ways to do this, all that I have found involve a slight bit of lag as the mouse click is sent to each client (assuming multiple clients on 1 machine).
Broadcasting to multiple machines removes the lag of having to cycle through windows, but obviously running that much hardware for AoE spell targeting is not an option for all