I've got a problem with simplexml_load_string() PHP5 function: it doesn't execute on php4. My hosting provider runs php4 which doesn't support simplexml_load_string. I can't install php5 because I'm on a shared hosting.

Is there anywhere a php4 compatible function or script that would replace this function?

simplexml_load_string() only works in PHP5 and above.

Considering PHP5 is stable and PHP6 is coming out soon I recommend you move hosting provider, clearly they are wasting your money.

However, their are PHP4 compatible XML functions (though they are not as good as the ones built in):
http://whoooop.co.uk/2005/03/20/xml-to-array/ - XML to Array Script (Should work in PHP4)