Gregory Szorc has posted his look at the "many untapped features" that PHP has to offer that the masses can tend to overlook when developing applications, specifically when dealing with object versus procedural programming.
The more interesting topic is why these applications have not made the jump to utilize PHP 5's features. Others have speculated, and I tend to agree, that application developers are worried that PHP 5 adoption is too low and requiring its use will turn away users. Now, considering the improvements of the PHP 5 engine, both from a performance and security standpoint, there is no reason in my mind why a sane system administrator wouldn't be running PHP 5.2.1 (most recent at the time of this entry).
He looks at applications like WordPress, Drupal and Gallery and note how they haven't made the move to true PHP5-style code. Adoption has a lot to do with this, but he brings up the point of the article - that developers are "ignorant" to what they're missing.
I highly doubt more than 30% of the servers out there allow php5. I'd also say 80% of the users are "non-programmers" who can play with the software enough to get it working. So it really does not benefit the end user, and it seems that the developers are fine using php4.