RetroPie 4.0.2 images are now available. This is primarily a bug fix release, but also includes Dolphin – a GameCube / Wii emulator (only available for those running RetroPie on a x86/x86_64 machine).
You can download a 4.0.2 image from: https://retropie.org.uk/download/. For first installs please follow the Installation Instructions. If updating from 4.0 you should make a backup first then choose Update all installed packages from RetroPie-Setup main menu. Anyone upgrading from 3.x will need to update the RetroPie-Setup script first. See the upgrade instructions on the Wiki for more details.
You can also install RetroPie on top of an existing Raspbian set up, or on top of Ubuntu on a PC/Odroid-C1. Links to the relevant instructions can be found on the Download area.