SLI / XF let me run 4 screens

From what I've read...the new and coming Nvidia Fermi cards will run 3 monitors from 2 cards sli'd and ati eyefinity can run 6 monitors now.

Yes, I am currently running 4 monitors with a 4870x2 and a 4870. Keep in mind though that you won't be able to use Xfire AND 4 monitors at the same time (only 2). What works for me is having one parition that I just use for games, and another that I use for everything else. I have Crossfire always enabled in the game parition, and always off for the applications parition (you could just enable, disable crossfire as needed as well)

so will two 4890 let me run 4 screens

yes. you'll be in the same situation as mine. just not be able to use 4 in Xfire, only 2.