PHP has already a long history behind him : Legendary PHP 1.0, PHP/FI, PHP 3.0 and PHP 4.0.
Migrating from PHP 3 to PHP 4
PHP 4 was designed to be as compatible with earlier versions of PHP as possible and very little functionality was broken in the process. If you're really unsure about compatibility you should install PHP 4 in a test environment and run your scripts there.
Also see the appropriate migration appendix of this manual.
Do sessions work in PHP 3?
Since PHP 4 is basically a rewrite of the entire PHP engine there were very few functions that were altered and only then some of the more exotic ones.