4
you're prolly gonna be outta luck with most CMS's, cus from what i've seen, XOOPS v1.xx might possibly work with php4.0.6, but wasn't very good imho(that's why i didn't go with XOOPS when i was originally looking for a CMS), phpnuke has too many security issues and bugs, but once you apply ALL the patches, it might be worth trying to see if it works with php 4.0.6
nsnnuke is a much nicer branch of phpnuke, more security-oriented, better interface, etc, but is still in beta-phase
http://opensourcecms.com/ << look here, demos of tons of different cms's, test them out, find some that you like, and check with the developers of that cms for 4.0.6 support
or you could wait and see if the XOOPS dev have a suggestion for you, cus if you can get a version of xoop2 to run on php4.0.6, that would be the best way to go