You can configure it to run multiple versions, in fact I did some research for a collegue on how to do it last year...
Myself, I find it overkill... Want to know why PHP4 scripts commonly don't run in PHP5? They where coded assuming that register_globals is "on", whereby they're set "off" by default in php5 and removed completely in php6 due to potential security issues.
How easy is it to fix? it's trivial, if you're reading a post or get variable, you need to define the variable vs assuming it's always there... Takes all of a couple of minutes to update...
