As far as I know there is no time line for RetroPie on RPi 4. At the moment RPi 4 is very new hardware and the release date was a surprise. It was planned for end 2019 or beginning of 2020. RPi 4 has a new CPU/GPU/VideoCore and USB3/GBit Ethernet/2 x 4k HDMI and other new things. They still do some tweaks for the official supported OS Raspbian. Of course for the time you can try the current RetroPie version or one of the unofficial releases for RPi 4 to see how it works. An update later should be possible. Then there is
https://www.lakka.tv/get/linux/rpi4/
Maybe you want/need to update the RPi 4 firmware first:
https://www.raspberrypi.org/forums/v...90467#p1490467