When did people decide to stop using correct grammer and spelling?
If you're on Windows ignore the tutorial - its for a Linux install and even then it can be overkill for a quick solution. XAMPP for Windows (or Linux) lets you switch between PHP versions. Another solution is to edit httpd.conf (the Apache config file) to use both PHP4 as CGI and PHP5 as Module. You can then create a separate directory assigned to a different localhost port from which PHP4 in CGI mode will be used. All other files in htdocs will then use PHP5.
Now if I can remember the url to a pre-existing tutorial for this...
This looks likely a candidate - you can switch around which uses the special directory and port if you wish:
http://circle.ch/blog/p1387.html
This would result in PHP4 running as module parsing files from:
http://localhost/
And using PHP5 to parse from
http://localhost:83/
Where Port 83 maps to a subdirectory of htdocs - specify the location in the config for the virtual host.
All you need is a standard Apache/PHP install, and the Windows Binaries for the second PHP version you intend using - in the tut above its PHP5.