Steps to reproduce the issue

Do a clean install of J4 and then try to login in frontend using the module with wrong credentials. Also try the two links for forgotten user and password.
Even with SEF turned off.

Expected result

Works

Actual result

Gives an 404 Invalid URL error

Additional comments

As soon as you create a menuitem for com_users, it starts to work. So it looks like the same "issue" with com_content. You need at least one com_users menuitem, otherwise the router just refuses to work.
We at least need to create such a menuitem during installation so the base install actually works, but I seriously think the router needs some sort of fallback or many sites will face issues. Telling them they need to set up their site "correctly" isn't helpful.