php4 and php5 are different in the architecture. php 5 is written on zend 2, while php 4 still utilizes zend 1. to know about the difference, you can:
1. rtfm (f= friendly)
2. read other's userland code.
3. read the (c) source code(cvs.php.net)
fyi, you can run apache with both php4 and php5 on your server. john ever wrote an article about this. (i forgot where, maybe in onlamp.com or coggeshall.org).
happy reading!
