Its because PHP got really famous with version 4.0 and many people actually converted their CGI or other websites in to PHP 4 websites because it was easy and cheap. But 5.0 brought too many changes like serious OOPS and register global concepts for security, which is useful but made transition difficult. I feel thats why PHP 4 is still supported.
Its not only the language that has changed, but also people had to upgrade their skill set and there was some learning curve involved. Unfortunately everyone fell in the trap of register globals which was not dealt until php 4.3.1 as a security concept. Pear and Pecl were there but everyone was pretty much writing all the code (reinventing the wheel) from scratch. This brings in huge code base to change. I liked PHP because intitially it was a procedural langauge and it resembled C. But now with OOPS you can build powerful websites which is good. There are many other cases but I feel strongly this is what makes them still support PHP 4. Thanks
