[support] PHP 4 or 5?

[C. R. Oldham]

Greetings,

I was a PHP user from way, way back when it was still known as PHP/Fi.
Various things have taken me away from it, but I am now going to migrate
some of my work to Drupal.

I am having trouble making the choice between PHP 4 and PHP 5.  I use
Debian, but often roll my own packages or use www.backports.org.  Can
someone with more recent experience than I comment on what pitfalls I might
face if I choose 5?  

--cro