I was wondering if there was a way (using easyapache) to allow my customers to choose wether they want to use PHP4 or PHP5 on their domain, or is it strictly global inside of WHM?
Yes you can install both PHP4 and PHP5 on your server and to switch over to PHP versions you will have to login to cPanel for a particular domain --> Click on PHP Configuration and select the php extension from the drop down menu and click on Update.