I was reading Nick Lewis post on why “Should Drupal move to PHP 5″ and personally don’t agree much with the reasons that PHP4 is dying, and the days will prove that it will not very soon. However there are lots of excellent reasons to switch to PHP5, or even PHP6.
The advantage of switching to PHP5 will be first a much higher quality code with full OOP support, then it will be faster for development and maintenance, and more interesting is the availability of enterprise frameworks. But technically for the endusers there is not much change, while your application might run faster with PHP4 (at least now), you will have to think twice before dropping totally PHP4 support.
For hosting companies I mentioned previously why supporting PHP 5 will be a wise decision, and I think that decisions to make popular open source projects switch to PHP will certainly give hosting companies another good reason to support PHP5 in their hosting packages.
And in real world, as Steven Wittens explained very well in comments, it is better currently to not drop PHP4 support and develop a PHP5 version hoping to drop one day the PHP4 support when it will be dropped also from PHP.net officially.