4.0 is a superset of older versions of framework but it is independent of 3.5 or any other older frameworks. It is a standalone package.It is independent due to its brand new CLR.
The framework is constantly reviewed by MS. If you see the evolution of the framework, the 2.0, 3.0, 3.5 had been on the same CLR but it was much different from previous 1.1 Framework. The versions which followed 1.1 had vast modifications. And, now 4.0 has comeout as a new offspring as shown below:
There are various products such as Visual Studio 2010, SPS2010, BizTalk 2010, etc.. which are already based on this new 4.0 framework.So fire up your engines and try your hands by downloading the latest .Net 4.0 Framework Offline Setup package from this link download