Will RC3 contains the MinGW 4.x? instead of 3.x?
That can safely be answered with "no".
It is not decided whether MinGW will be included at all in the base installation, but it is certain that it will not be a 4.x version. The 4.1/4.2 branches are unluckily not production quality at this time, and the 4.0 branch is not sufficiently superior to the 3.4 branch to justify the problems that it introduces. Also, all 4.x versions need to be compiled from sources, which is tedious and time consuming (on the order of hours, rather than minutes).
The likely thing you will get (no guarantees) will be a base installer without any compilers, and a MinGW package for the builtin web updater, so users can install MinGW with a few clicks when first running Code::Blocks.
Maybe, possibly, there will be a convenience package that already comes with MinGW preinstalled, too, but it will certainly not be 4.x.