Very bad idea to have to touch your customers code. That could end up being a true nightmare scenario and for free - thats just a bad idea.

Offer both and encourage your clients to migrate to the new platform. As far as PHP 4 not being supported - anyone who has been in this space for a few years knows that people do not have the time or inclination to fix projects that are not broken even if substantial advantages may exist. Legacy PHP 4 is going to be around for awhile and nudging your customers to make the switch is the best business solution.