Originally Posted by k0nr4d

Well, personally the main advantage of 5 over 4 for me is the additional functions. SimpleXML for example, makes parsing xml significantly easier. The addition of file_get_contents/file_put_contents (I forget which was added in php5) has also made it much easier to work with flat files.

Additionally, php4 has had its end of life announcement, and as of the new year no additional versions of php4 will be released.

The only incompatibilities between scripts I've seen for php4 not running under php5, is due to register_globals being off by default in php5.