Hello fellow 1and1 users:

I have VB installed at domain.com/forums/ and after upgrading to 4.0 suddenly my .htaccess wasn't sending domain.com or www.domain.com to the /forums/ anymore.

After way too much trial and error I found this code worked for me:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^domain\.com$ [OR]
RewriteCond %{HTTP_HOST} ^www\.domain\.com$
RewriteCond %{REQUEST_URI} !^/forums/
RewriteRule (.*) /forums/$1
It looks nothing like the code I had before but it works now so I'm not asking any questions.