Not sure about Mac OSX.
Linux very easy.
1. Get PHP 4.3.x source, and ./configure as necessary (extra args you need), make, make install
2. Access httpd.conf (/etc/apache/httpd.conf on slackware for instance) and comment out the php5 LoadModule and AddModule calls (the make install should have already added the LoadModule calls for php4). Save httpd.conf (Edited).
3. apachectl stop, apachectl start (or however else you restart apache).
4. Enjoy.
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