As we already mentioned, the Athlon64 4000+'s basic architecture is the same as the Athlon64 FX-53.
This is why all the technical specs are identical: Sledgehammer core, 2.4 GHz, 1 MB L2 cache, Socket 939 and 89 Watts Thermal Design Power. Things like Cool & Quiet and NX-support are also available.
Releasing the 4000+ is a smart move as it does not cause any changes in the current 130 nm production. Only the labeling has changed compared to its predecessor.
Regarding the FX-55, obviously AMD has achieved a better-than-expected yield rate with its 130 nm process as it has managed to launch a 2.6 GHz device based on 130 nm. Previously, AMD was not expected to offer devices at this speed prior to starting up its 90 nm process.
As the non-FX CPUs reach better sales due to more attractive pricing, the former FX-53 is simply transformed into a 4000+. Using the Newcastle core with 512 kB cache at increased clock speed hence is not necessary and leaves production capacities for lower-speed versions that are desperately needed in order to make Socket 939 affordable for average users.
AMD did it again: After turning Athlon XP into Sempron, the FX-53 now will be the Athlon64 4000+.