Support for PHP4 has been discontinued since the last day of 2007; there are some Drupal modules that shows an image suggesting to pass to PHP5, and Drupal 7 (which will be the only to implement new features) is for PHP5 only.
In spite of this, many web sites still use PHP4, and for many of them there will not be a passage to PHP5 in short time; for many of them, the decision of changing for PHP5 is not taken from the webmaster whom must wait for the company which offer the web hosting service that will pass to PHP5 just when some conditions are met.
I am curious to know how many people are using PHP4; if you want to help me in such research you can vote on this poll.
From my side I can say PHP5 is installed on my Mac OS X computer, but simply because it comes with the OS. I would have installed it, if it would not have been installed, but in this case I have no merit; if the OS would have had PHP4, I would not have installed PHP5 and have two copies of PHP running on my computer.
A note about the poll.
As the poll accepts just a vote per user, vote for the version of PHP you use almost all the time, or the version of PHP you develop for most of the time (if you are a developer). If Advanced Poll would be available for Drupal 6, there would be the possibility to vote for more than one version of PHP, but at this moment there is just the core poll module.