Normally one makes a backup of these sort of critical system configuration files. I take it you haven't got an httpd.conf.bak or .orig or anything of that nature?
If not, I'd recommend force-killing Apache using whatever the mac equivalent of 'kill' is in Linux, or use a graphical system process list of some kind. But anyways, kill off all instances of Apache, and re-configure your httpd.conf file - maybe find a demo one you can get online and just over write it, or go through and comment out the php4 module lines again?