Screen tearing is a issue of the Raspberry Buster Lite that the current RetroPie image is using. The upcoming version i think it will get rid from this issue.
Some users solved this issue, in the current version of RetroPie, after they use the experimental kms driver, but this add other issues. If you search in the forum about screen tearing you will find a lot of info. Also i recommend to read the wiki about the used of 4k TV with Rpi4: https://retropie.org.uk/docs/Pi4/
