PHP 4 is officially dead and only being updated to address serious outstanding security issues, however if you use a lot of cheap/free code from the internet that uses any of PHP's OOP features, odds are very good that it will no longer run in PHP 5.
Jumping from mySQL 4 to 5 typically involves manually resetting a lot of passwords. If you're not comfortable administering mySQL via command line then it's not something I'd recommend bothering with unless you know that you need some specific new feature of 5.