How do I edit the .htaccess file to point to a relative directory for a 404 error document?

Most htaccess 404 error rules are based on an absolute directory location, eg /404.php. I want one that is relative to the location it's placed in (so when I transfer from test environment in WAMP to live, I don't have to hope I edit the file right).

Unfortunately,

ErrorDocument 404 404Error.php

Just prints out 404Error.php to the browser, it doesn't call the actual error page like

ErrorDocument 404 /FOOBAR/404Error.php

does.

Edit:

I guess I wasn't clear enough. Test environment is a WAMP local server, with the error file at /FOOBAR/404Error.php while the 'live' server would be www.fubar.com/404error.php -- placing it in the root.

How's this?

ErrorDocument 404 ../directory/404page.html

RewriteBase /FOOBAR/
ErrorDocument 404 404Error.php

This works too, but Tycho's solution is a bit more elegant. Only use this one if you don't plan on using .htaccess for anything outside of /FOOBAR/ pretty much.