Michael Kimsal has posted a formal prediction that a patch, created by someone in the community, will emerge making it simple to bridge the gap between PHP4 and PHP5 applications.
Given the high number of shared hosts still running PHP4, the backwards compatibility issues going from 4->5 (yes, there are some!) and the new PHP4 "end of life" date (8/8/8), someone will likely find a way to make these run together, and possibly even charge money for it.
He suggests that many hosts out there would happily pay a sum to get this kind of functionally for their customers. It would provide a solution to customers who just don't understand the force towards PHP5 and would make them (and their PHP4 applications) happier.