The PHP development team has announced that support for PHP 4 will
continue until the end of this year only. After 2007-12-31 there will
be no more releases of PHP 4.4. Critical security fixes will be made
available on a case-by-case basis until 2008-08-08. For documentation
on migration for PHP 4 to PHP 5, there is a migration guide. There is additional information available in the PHP 5.0 to PHP 5.1 and PHP 5.1 to PHP 5.2 migration guides as well.
Source: Slashdot