Name what you want to run in PHP4 with the ".php4" extension and what you want to run in PHP5 with the ".php5" extension.
So in your case name everything on your subdomain with the ".php5" extension and everything else with the ".php4" extension.
From what I read this is a new thing in WHM / cPanel and is highly experimental but if you want to give it a go It'll be interesting to see you get it up and running
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