XML development

So I’m planning some development in PHP that will use XML and DOM. Here is what I am wondering... Should I use PHP4 or PHP5. PHP 5 has much better support for XML and would probably make my life easier, but it may or may not be on all the various servers that I may want to use it on. This will be a library to do some stuff that I plan on reusing for a lot of customers, and I don’t know where they all plan on hosting their web sites.

Any thoughts?