Is there a performance difference between php4 to ...

PHP5 is not significantly faster, from what I remember it's actually slower than PHP4 so if performance is important to you I think you should stay with PHP4 for now. But PHP4 is nearing its end of life, and support for it will be discontinued in the end of 2007, so you should start thinking about moving to PHP5 pretty soon.

One of the big advantages of PHP5 is its support in object oriented programming. This allows for better coding as well as having a very slim namespace support (what's inside the class is just there).

Upgrading is worth it just for the OO feature and having clean and maintainable code.