PHP 4.0 support

Hi, I just got a notice from my host that I have seven days to update my domains to use PHP 4.0. I actually have multiple website clients, as well as several domains of my own, using Wordpress with various versions of Atahualpa, some of which go back to before there was an "Export Settings" feature. I'm concerned that some of these might break after a PHP upgrade. I'm pretty sure all, or nearly all of my clients are using recent versions of Wordpress, and Ata still works with those, but how can I tell if this upgrade is going to break something? I have no idea how to find out which PHP versions are supported by which Ata versions. Is there a table somewhere?

Thank you!