Apache com duas versões do PHP

sim..

basta espcificar o módulo dentro do virtualhosts

para isso, obviamente, precisa trabalhar com virtualhosts para cada host

exemplo ( dois hosts, foo.bar com php5 e bar.foo com php4 )

NameVirtualHost localhost

<VirtualHost foo.bar>
 LoadModule php5_module "C:\php5\php5apache2_2.dll"
 PHPIniDir "C:\php5"
 php_value extension_dir "C:\php5\ext\"
 AddType application/x-httpd-php .php
 AddType application/x-httpd-php-source .phps

 ServerName foo.bar

 <Directory "C:\_w\foo.bar">
 allow from all
 options +Indexes
 DirectoryIndex index.php index.html
 </Directory>
</VirtualHost>

<VirtualHost bar.foo>
 LoadModule php4_module "C:\php4\php4apache2.dll"
 PHPIniDir "C:\php4"
 php_value extension_dir "C:\php4\ext\"
 AddType application/x-httpd-php .php
 AddType application/x-httpd-php-source .phps

 ServerName bar.foo

 <Directory "C:\_w\bar.foo">
 allow from all
 options +Indexes
 DirectoryIndex index.php index.html
 </Directory>
</VirtualHost>