There's not much that one can do to help since the security fixes may have to be backported (and that's if PHP itself is not the problem, I haven't checked). For the time being you can configure your wiki to ignore the warnings [1]. PHP4 itself is officially deprecated by now anyways, so if your server can't move from PHP4 to PHP5, that's entirely another issue (of another degree).
[1]
http://www.dokuwiki.org/update_check
If you really need to backport security fices and the like, and since the main dev team won't be working for PHP4 anymore, I'd say your best chance would be to either find other users who are sticking with PHP4, or
set up a bounty.