That code is PHP5-only. If you've written much code like this and want to run it in PHP4, be prepared to spend a lot of time hacking away to make it work
PHP4 didn't have all the fancy OOP stuff like PHP5 does. You'll need to change that "protected" to "var" for a start. Any constructors need to be change from __construct() to the name of the class itself. Any interfaces or abstract classes need to be dropped. Any static class fields need to be removed. Any destructors won't work. But most annoyingly, PHP4 doesn't handle object references correctly so you need to use the "&" reference operator everywhere to explicitly create references to objects which don't behave the same as proper object handles in PHP5.
Fortunately for hosts who upgrade to PHP5, and for developers who wrote PHP4 code, PHP5 is backward compatible with PHP4 code, but not vice-versa.
I'd suggest find a new host or asking your host to upgrade since PHP4 is out of development as of December 2007.