Welcome aboard. FWIW, PHP 5 is not available on a lot of web hosts, yet, so watch the documentation when you code if you choose a 4.x.x host. The latest release, 5.2.1 seems to be pretty stable and maybe we will see more hosts moving to 5. There was a fork in the code and PHP just released 4.4.6 (after 5.2.1).
PHP 5 added some great ideas, but unfortunately the bug list was huge,
http://www.php.net/ChangeLog-5.php it broke some 4.x.x code. So a lot of ISPs have been slow to move to 5.x.x for fear of breaking customer sites.
I hope my ISP will offer 5 soon.