after changing the main.php file from dso mode to cgi mode i have problem with one of my site which uses php4 in my server. my server has dual php (4 and 5), but the trick for working all the sites which nedd php4 i was used to have in the .htaccess file of each site these line
AddType application/x-httpd-php4 .php

unfortunately when i changed to cgi mode,one of my site(which uses php4) was not responding and i was geting internal server error.when i delete the .ht access file, site worked but one of my serious script(it is a chat script which uses php4) stopped working.

can you help me?please advise me!

What error were you getting in your server logs that corresponds to the Internal Server Error?