I also accidentally installed it. I didn't realize that tvOS only has one beta, unlike iOS which is a different beta program for each major version. and forgot that I had beta updates turned on. So woke up one morning and found I had tvOS 13.
It's been fairly stable for me so far. and there's a new version out yesterday, so maybe that will fix whatever problem you have
a quick search for "downgrade tvOS beta" and it seems that you can't downgrade the 4k, since it has no USB port