They are cutting support for PHP4 about 11 months from now, so anyone who wants to continue programming in PHP will learn 5. Luckily there are a lot of similarities, so if you have any familiarity with PHP4, you'll be alright transitioning to 5.
XAMPP is a great program, it's what I use for web development. If I remember correctly, the newest version has a batch file to switch between versions 4 and 5, correct? The big changes between 4 and 5 are updates to security obviously, and a total OOP structure makeover. Anything else REALLY significantly different between them?
Yes, it does! Isn't it great the way it makes learning PHP/MySQL so much easier for the rest of us? Cheers!