PHP 4 has been arround a long time, since 2000 actually. On July 13, php.net announced that PHP 4 (which has been in maintenance-only mode for 3 years) will be discontinued, because PHP 5 has been available for 3 years and PHP 6 is on the way.
The adoption of PHP 5 has been slow, but there are initiatives starting to show up like GoPHP5.org that have set a date (February 5, 2008) on which a list of major PHP based programs will no longer support PHP 4.
Also, other signs signify the end of PHP 4, like in a newsletter from my webhost that they also will switch over to PHP 5.
Guess I better check my scripts on PHP 5 compatibility 😉