PHP4 is officially "end of life" by PHP, and is deprecated here at DreamHost. There is no way, at present, to set a domain to run PHP4 on DreamHost other than to compile your own PHP4 PHP-CGI.
Old domains that had PHP4 active we left grandfathered for a bit, but once a domain is set to PHP5, it cannot be reverted.
The old workaround of using mod_php PHP4 via an .htaccess directive is now no longer working either. As servers have been upgraded, mod_php has been removed altogether.
It is, quite simply, time to say "Goodbye" to PHP4.
--DreamHost Tech Support