If you receive a Grav-style error saying Error 404 then your .htaccess is functioning correctly, but your trying to reach a page that Grav cannot find. The most common cause of this is simply that the page has been moved or renamed. Another thing to check is if the page has a slug set in the page YAML headers. This overrides the explicit folder name that is used by default to construct the URL. Another cause could be your page is not routable. The routable option for a page can be set in the page headers.