NOTE: RetroPie has been released for the Raspberry Pi 4 since these were provided and posted so if you want the latest, stable build then you can download that here.
Below you can find all of the weekly builds for the RetroPie project on the Raspberry Pi 4. The latest version is at the top and the builds share version numbers at times so make sure to download the latest (unless you want to test a specific build!)
Links: Official Mirror – Reddit Feedback Thread