how to enable php4 only, no php5
Solution 1:
There are no PHP4 package in Ubuntu 9.10.
I guess you could download the PHP4 source from http://www.php.net/releases/ and compile it yourself.
Myself I would download the still supported Ubuntu 6.06, which does have PHP4 packages, and install it in a virtual machine. That way I could play around with old stuff without causing conflicts with my current system. You can find Ubuntu 6.06 iso files at http://releases.ubuntu.com/6.06/