According to an official post on Samsung’s South Korean forum, there will be no more Android 12 beta for the Galaxy S21 series. The fourth beta version is the last before the sharp release.
It should be good for both because the work is almost complete and Samsung will start sending out Android 12 within a few weeks. The sharp version should fix the last bugs found in the fourth beta.
Samsung sent out the first beta version for the S21 back in mid-September, so the system has been publicly tested for almost two months. According to a previous statement, the S21 trio should be updated to Android 12 by December.