I think doing this within WordPress is probably the preferred approach, as mentioned in @Harrison’s answer, however, to answer your specific queries…
RewriteEngine On Redirect 404 /article.html
Aside: The RewriteEngine directive relates to mod_rewrite, however, Redirect is a mod_alias directive – so these two directives are unrelated.
However, this should “work” with a default WordPress front-controller, so maybe you have a conflict with other directives in your .htaccess file?
Try the following instead, using mod_rewrite at the top of your .htaccess file (before any existing WordPress directives):
RewriteRule ^article\.html$ - [R=404]
If this still results in an error, then try resetting the 404 error document (to the Apache default) before this:
ErrorDocument 404 default
You’ll need to clear your browser cache to clear the cached (permanent) 301 redirect to the new URL.