To chime in, I’m having the same problem.
However, it looks like the plug-in just doesn’t know where to find install.php by default. For example, my install.php is in http://…/wp-content/bbpress/bb-admin/install.php but the 404 error comes when the plug-in looks for it in http://…/wp-content/bbpress/install.php
I’ve found that manually rewriting the URL to the true install.php will side-step the step 1 404 error.
