Mapping several URLs to one login page
The last rule is completely superfluous, but harmless: it just maps the desired URL to itself.

The four cases can be combined into one rule:

RewriteRule ^(_user/)?login(\.php)?$ _user/login.php


In a regular expression, (something)? makes the something optional.  That is, whether the URL starts with _user/ or not, and whether the URL ends with .php or not, it will get mapped to the desired login page.