Phil
The best way to do this is to look at the PHP Changelog and look at the changes made in the 5.0.5 release (which fixes the bugs in 5.0.4). The bug fixes which jump out re security are the ones related to safe mode and sessions. There are some mysql ones also which forms part of the "functionality" requirements that your host has.
Possibly a repost in the developer forum may get you some very specific reasons.
HTH
Ken
Hardware and performance
PHP 5.0.4 or PHP 4.4 for Moodle 1.6
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