When I started out PHP'ing, I bought a little PHP book called 'PHP pocket reference'. It was one of those small O'Reilly books and it was written by Rasmus Lerdorf himself. I still have it, although of course I don't use it anymore.
The book focused on PHP3, although PHP4 was already released. So, basically, I learned PHP with version 3. I started out writing scripts with a .php3 extension, something I still don't understand; why was the version number included in the extension?
Anyway, not long after I started PHP'ing, my environments switched to PHP4 and I became a PHP4-developer. And I still am. One of my employers still uses PHP4 and although today is PHP4's death date, there is no indication that there's an urge to speed up the upgrade process (which thankfully is in place and being worked on).
The problem with PHP4 and PHP5 is that the upgrade process actually is a big step. For the software, because not everything 'just works' when you upgrade to 5, but also for the developers. Some of my co-workers still consider the features that were introduced or improved in 5 'new', although they've now been included with PHP for years. And that's understandable; when working heavily on projects in PHP4, and without having the opportunity to check out what's ahead and trying to use newer features, you'll never get a taste of it. Of course, developers should take those opportunities themselves, checking out new features and developments in their own time, but not everyone does that.
So today is the end of PHP4. Not really of course, because lots of developers will probably still spend months working on PHP4 code. It will work just fine and do just what you want forever, it just won't have any updates anymore, But if you consider the fact that some servers (even those of my employer) still run a 4.3.X-version of PHP, that hardly matters.
I'm glad I switched the projects at my other employer to PHP5 at the start of 2008; not only are we up to date, but the new features (or simply the small improvements in existing features) make working with PHP a lot nicer. And we're ready for the future; PHP6 is upon us, and I hope it will be adopted (and adoptable) a lot faster than its predecessor.
PHP4, you've served us well. You paved the way for PHP5. Thanks! Now get out, and stay out.