On top of that there is the upgradeable factor.
This really only makes a lot of difference if you don't already have any of the cards, but later down the line, when tow 4890's or one 5870 aren't enough for you (new dx11 crysis 2 comes out maybe). upgrading the 5870 to dual 5870's would be easier and better than three or four 4890's.
the 5870's are just newer better hardware and since the prices are close it just makes sense to get it. they will last longer in the computer world, and the 4890's will become worthless sooner.