For those of you that still only want to get 14.0.4 RC 1 and not 15.0.0 Beta 2, we have set up a special updater server only for this purpose. You can configure it in the config.php:
$CONFIG = [
'updater.server.url' => 'https://updates.nextcloud.com/14.0.4rc1-on-beta-temporary/',
];
Once this is configured, your Nextcloud will show 14.0.4 RC 1 instead of the update to 15.0.0 beta 2 in the update section. Keep in mind to remove this config option from config.php after the upgrade was done to use the default update server again, because we will remove this once 14.0.4 is out.