For some reason the MinGW project has been very slow to release a GCC 4 version. At first they said they were waiting for 4.1 but that arrived and still nothing. They eventually did release an experimental build but there hasn't been much action on this.
Checking the status recently, I searched for MinGW 4 and discovered the TDM versions of MinGW, including 4.3, and they seem to be active with it.
In a way this is a "fork" of the MinGW project - one of the benefits of open source. But it's not a fork in terms of going in a different direction, only in terms of releasing sooner. I really wonder why the main MinGW project does not want to release version 4, maybe they have a good reason, but if so they don't seem to be communicating it very well. Obviously they could still maintain version 3 if they feel that is a better choice for some things.
I built Suneido with the latest TDM version. GCC 4 has a lot of improvements to optimization, but sadly it's still slower than Visual C++ (about 10% slower than VC9, which in turn is about 10% slower than VC7)