Regarding MV4: the WebExtensions working group has started to mention the term at some of their recent meetings, although details are scarce it seems MV4 will still retain JS code execution [1] [2].

[1]: https://github.com/w3c/webextensions/blob/main/_minutes/2022... (CTRL-F for "mv4")

[2]: https://github.com/w3c/webextensions/blob/main/_minutes/2022... (CTRL-F for "mv4")