You guys are missing the obvious answer...
We'll support PHP4 as long as squirrelmail does. :)
In all seriousness, unless the developers want to use PHP5 functionality I don't see any reason not to continue supporting 4 for as long as the PHP team does.
On the other hand, if the devs want to use PHP5 in all it's OOP goodness, then toss PHP4 support in the river and never look back.
For those that absolutely need PHP4 support, the current version works great based on the reports of people using it here and if a security problem comes up everyone has access to the source.
It would be as simple as branching and only doing security fixes to the PHP4 branch.
Maybe this could happen at 1.0.
Hope this helps! Matt