But why develop in PHP 4 at all, any more? PHP 4 is dead and the dirt is being shovelled on as we speak. There would have to be pretty compelling reasons to still be avoiding 5 after all these years. Not only has PHP 4 been stagnating for a long time now, but with cessation of support upgrading will only get increasingly more painful the longer it's delayed.
And if you say it's because "my host doesn't support PHP 5", get a new host - assuming that you're paying them.