It should be able to power 4 monitors in Windows, but if you plan on gaming on multiple monitors with eyefinity, this card can only do 3. Although that is a bit moot, because this card doesn't quite have the power necessary to power more than 1 monitor on modern games unless the resolution per monitor is low.
UPDATE: Actually, AMD says you can do 6 gaming, with additional adapters from miniDP to dual DVI or HDMI outs. BUT, you need a reference card. If you plan on doing this, make sure you check to see if the card you are about to purchase is a reference design or not.