You can do it by building two apache modules with different types: "application/x-httpd-php4" and "application/x-httpd-php5". Load them both into apache, and specify in
AddType application/x-httpd-php4 .php4
AddType application/x-httpd-php5 .php5
AddType application/x-httpd-php5 .php # the default
Now you'll have to use ".php4" extension for files to be handled with PHP4, and ".php5"
for PHP5. ".php" files are also processes with PHP5 by default.
UPD: In order to have some folders executing PHP scripts with PHP4 module you can override the default PHP5 with PHP4. Just use this in
AddType application/x-httpd-php4 .php
With php in CGI mode you'll be able to use
.htaccess configuration. Build both PHP versions in CGI mode, place the executables into
cgi-bin folder, and try the following in
AddType application/cgi-php php
Action application/cgi-php /cgi-bin/php4.cgi
However, this way is much worse in the means of performance.