php-cgi limitations

Can anyone offer or point me to a summary of differences between running php as an Apache module vs as a cgi executable? Specifically, I'm wanting to upgrade our servers to php 5 but there is one crucial third-party app we're using that still needs php 4, so I'm hoping I can run just that app under php 4 cgi until the vendor updates to support php 5. Google is unusually laconic on this, but perhaps I just offended it with too many daft queries on the subject... :) -- Max Schwanekamp http://www.neptunewebworks.com/