@Mark Murphy To your first question, Leland was referring to the docs/AS Preview page, which has since then been updated to have a link to both the beta and canary versions of AS.
To your later question about AS 4.0 and compose, 4.0 will not support Jetpack Compose. What will likely happen is that Compose functionality will remain in canary builds for the foreseeable future and when a beta is cut, it will be moved to the next canary version. So when 4.1 is ready for a beta release, it's likely that Compose functionality will be stripped for the beta, and there will be a AS 4.2 Canary build that supports Compose. If you want to do Compose testing, you'll want to ensure you stay on Canary builds, and don't upgrade to a beta build.