Many reviewers have proven that the Sledgehammer core for the Opteron is architecturally exactly the same as the Newcastle, Winchester, and Clawhammer cores for the Athlon 64. It is just AMD's way of squeezing more money out of people. Since they are gaining more of the market, they believe they can start dictating prices like Intel can right now. (No Offense) The 4000+ is just a renamed Athlon 64 FX-53 except it doesn't have a multiplier that is fully unlocked. The Athlon 64 4000+ has a multiplier that can go down, but it can't go up. If you're ocing the the FX-53 will the better of the two.
Oh and I think Apokalypse is right about the 4000+ 130nm and 90nm versions. It seems logical to me because the 90nm processors have a 7% increase in speed over a 130nm with the same rating. So, it is logical that AMD would trade in 512K cache for 90nm since it would mean better perf on the 90nm, think about it you oc a 90nm by 100 mhz and your actually getting a 2.675 clock speed instead of 2.5 since the 90nms run 7 % faster clock for clock.