TweakTown has gathered $800 worth of 4850s together to provide you the following information.
In 3DMark06, 3DMark Vantage, and Unreal Tournament 3:
4 HD 4850s are better than 3, and 3 HD 4850s are better than 2.
In Cinebench R10 and all the other games that they tested:
4 HD 4850s are worse than 3, and 3 HD 4850s are worse than 2.