Well, it's not a full release, but a beta type release. MinGW has released GCC 4.2 on Windows is now available.
The first proper version 4 release will be 4.3 when it is released. This is because Windows will become a properly supported platform as historically MinGW have done loads of work making it work on Windows.
The jump from 3.4 to the version 4 codebase will provide massive improvements including OpenMP for multithreading, better and more powerful optimisations and better language support.