Take a look here:
http://www.howtoforge.com/apache2_with_php5_and_php4
http://www.howtoforge.com/apache2_suphp_php4_php5_p3
if you use the first example, you can enable PHP5 as mod_php or FastCGI from within ISPConfig, and then you add directives for PHP4 in the
Apache Directives field in ISPConfig.