Got a Lakka nightly build now.

Read a lot about RetroPie, gonna try Lakka first as you can use RetroArch, which I'm very used to by now. Don't know if you can install both, Lakka and RetroPie, as this is my first Raspberry, so I have to fiddle around a bit, still a newbie.

Anyway, at the moment there's not yet support of RetroPie for the Pi4. Have to wait.