There is no reason to run PHP 5 and PHP 4 on the same machine as PHP 5 is backward compatible with PHP 4. In fact, it would be "sloppy" to include both in SME server.
A better use of effort by the community would be to help push SME 8 to production.
Then why don't we currently include PHP5 instead of PHP4 now?
You must have spotted the reason: We must maintain as much compatibility with the upstream vendor as possible (SME -> CentOS -> RHEL).
I do not think it would be sloppy at all. It would make SME even more compatible with the wide range of legacy applications and the ones approved by RedHat and built specifically for RHEL. Anyways, I was talking about SME 8. We would include the RedHat version while also having the most bleeding-edge (read: most recent stable) for the end user to decide on.
It is most certainly not a bad thing to have multiple versions of the same product because in the real world, these things happen all the time.
Craig