I no longer code to support PHP4 on the simple principle that PHP 5 is now at 5.2.3 with 5 being available for over SIX YEARS. This isn't like supporting old browsers where you are at the whim of users worldwide - this is about your host. If your host hasn't joined us in this century or doesn't give you the access to keep up yourself (hell, even WHM has EasyApache that can handle it) you're on the wrong malfing host!
When working with inheritance of objects the 'new' constructor and destructors make developing object based code so much easier - I use that a lot when working with PDO when extending it. Abstract classes make building of frameworks simpler without exposing the base method to implementation, magic methods improve working with objects greatly - and I can't see writing my code to work without them anymore. Visibility levels on methods and properties can make your code more secure, Type hinting finally lets you have an error throw if the wrong data-type is passed to a function.
In my new code I've dropped using mySQL_ functions in favor of using a PDO, and keep my PDO as a local variable in a parent function, and pass it by reference to functions that would need it - preventing the connection or the connection info from ever being 'global'. The ability to take many methods 'private' and to make it throw if my custom extensions to the pdo object are fed the wrong values results in a much more secure codebase - and even though PDO is available in the later 4.x versions - I can't do a lot of that in PHP4.
Much less the slew of "new" functions many of which I'm using and would not want to go without or have to brute-force replicate.
Hell, there have been ZERO updates since 4.4.9 back in 2008 - and those were FATAL security flaws - many flaws remain in it unpatched because official support ended in 2007!
Unless you're supporting a legacy application that for some wierd reason doesn't work in PHP5, or for some bizzare reason have some complete trash host that's still stuck a decade behind the rest of the world there is ZERO legitimate reason to restrict yourself to 4.