There is no Joomla 4+ version of Simple User Notes.
Simple User Notes by Regular Labs is a Joomla! Administrator system plugin, that simplifies the management of user notes in Joomla.
In most cases, the built-in Joomla! User Notes system is overly complex if you just want to add a simple note to a user.
Simple User Notes replaces the core Joomla! User Notes system with a clean and simple textarea field in the User Edit View. This plugin is completely free.