How to get .htaccess to attach REQUEST_URI to 404 page

I don't think you have to pass any additional information to the error handler script, if that's what your question is about. Apache supplies enough information:

Redirecting to another URL can be useful, but only if some information can be passed which can then be used to explain or log the error condition more clearly.

To achieve this, when the error redirect is sent, additional environment variables will be set, which will be generated from the headers provided to the original request by prepending 'REDIRECT_' onto the original header name. This provides the error document the context of the original request.

For example, you might receive, in addition to more usual environment variables, the following.

REDIRECT_HTTP_ACCEPT=/, image/gif, image/jpeg, image/png

REDIRECT_HTTP_USER_AGENT=Mozilla/5.0 Fedora/3.5.8-1.fc12 Firefox/3.5.8

REDIRECT_PATH=.:/bin:/usr/local/bin:/sbin

REDIRECT_QUERY_STRING=

REDIRECT_REMOTE_ADDR=121.345.78.123

REDIRECT_REMOTE_HOST=client.example.com

REDIRECT_SERVER_NAME=www.example.edu

REDIRECT_SERVER_PORT=80

REDIRECT_SERVER_SOFTWARE=Apache/2.2.15

REDIRECT_URL=/cgi-bin/buggy.pl

REDIRECT_ environment variables are created from the environment variables which existed prior to the redirect. They are renamed with a REDIRECT_ prefix, i.e., HTTP_USER_AGENT becomes REDIRECT_HTTP_USER_AGENT.

REDIRECT_URL, REDIRECT_STATUS, and REDIRECT_QUERY_STRING are guaranteed to be set, and the other headers will be set only if they existed prior to the error condition.

None of these will be set if the ErrorDocument target is an external redirect (anything starting with a scheme name like http:, even if it refers to the same host as the server).

Check this link

I think you need to remove R=404. This worked for me;

Options +FollowSymLinks
RewriteEngine On

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule .* index.php?e=404&url=%{REQUEST_URI} [L]

Tests: foo/index.php : print_r($_GET)

localhost/foo -> Array ( )

localhost/foo/asdf -> Array ( [e] => 404 [url] => /foo/asdf )