Mageia 4 switches many of the inherited Mageia tools from being GTK+2 to now being GTK+3 applications, installation improvements, an upgrade to the RPM package management system, improved hardware support, and many other improvements.
On the desktop side of Mageia 4 is KDE 4.11.4, GNOME 126.96.36.199, and other alternatives like LXDE, Xfce, Razor-Qt, and MATE. Under the hood this Linux distribution is powered by the Linux 3.12.7 kernel, Legacy GRUB as its boot-loader (with only test support for GRUB2), Mesa 10.0.2, and syslinux 6.02.
The release announcement for Mageia 4 RC has yet to be published or announced from their blog but the release notes are available. The release candidate ISOs of Mageia 4 can be found via their mirrors.
The official release of Mageia 4 is expected in February.