What place is still running php4? The 4-series has been abandoned even for critical security patches some time ago, the move to 5 solidified a long time ago.
Maybe you're confusing PHP 5 and PHP 6; PHP 6 has been "just around the corner" now for a few years.
Is it? I was under the impression that there's very few hosts/libraries that don't have PHP5 support now.
PHP 5 was still, as much as I can remember, compatible with PHP 4, wasn't it? Anything that worked in PHP 4 worked in PHP 5.
That's not true. If one used OOP in PHP4, running that code in PHP5 will produce lots of bugs related to the new object model.