Don't change what isn't broken, sure, but what about changing what is antiquated? Think of it like this, you use something until it comes to the point where it is no longer efficient to use it. Say an old car. Sure, it still runs, but newer vehicles get better gas mileage. Therefore, it is no longer efficient to run the old car. The car served its useful life for several years, much like PHP4, and as such, PHP4 is being EOL'ed.
That all said, I'm (of course) a big fan of PHP 5.2. It is faster, has better support for OOP, making your code easier to reuse, and you have things like APC which is an opcode caching engine being written for it.
I can see the reasoning behind gophp5.org's movement, but I don't necessarily agree with it. Being the Support Team Leader of a rather large project, you have to support the largest audience possible, otherwise it is all counter-productive. Unfortunately, that means supporting PHP4 as well.