PHP 5

I noticed that PHP 5 has a lot of nice new features: SPL, SimpleXML, exceptions, PDO, etc. It does not seem to be widely deployed, however (from my limited knowledge). Does anyone have any opinions on whether it would be better to author for PHP 4 or 5?

Note: OpenSuSE comes with PHP 4.4, so I would have to compile PHP 5.1.