Try the following:

RewriteEngine On
RewriteRule ^/user/([^/]+)$ /documentroot/subfolder/user.php?userid=$1

You may have to tweak it a bit... I haven't done this for a while... the pattern might be ^user... instead.

OK, your rewrite was not done. The original /user/1234/ went through. You have a / on the end of the URL, which is causing it not to match your pattern. You can try changing your pattern:

RewriteRule ^/user/([^/]+)/?$ /hs/test.php?userid=$1

This says "Look for a URL (after the domain name) starting with /user/. Grab whatever follows, one or more characters, up to but not including a / and stick it into "$1". A / may optionally follow, and then the URL ends ($). Output a URL of /hs/test.php?userid= followed by whatever was saved in "$1".

Note that it is possible that the pattern will have to be tweaked to remove the first / (^user/...). Try that if it still doesn't match. Or, you could use ^/?user... to match either way. Also note: don't put [R=301] after the RewriteRule, as you don't want the browser or search engine to see that.

Go to cPanel > Error Pages and make yourself a 403.shtml and a 404.shtml (and 400, 401, and 500 if you want). That will get rid of the 404 Not Found error.

You're most welcome "a book"