Jerry, from what I have read recently, the x86 4p market has not been very large historically. However, the same article went on to say that since opteron scaled to 4p quite well due to its HT connections, that the 4p market was likely to experience a lot of growth. They even went on to postulate that the 4p market may replace the 2p market as the sweet spot for x86 servers.
Unfortunately the article was a few months back (I think about SUNW) and I don't have any links. You could google it though.