Our next ExpressionEngine release will mark the end of PHP 4 support in ExpressionEngine and CodeIgniter. For most of you, this should be a non-issue. PHP 4 was originally released in May of 2000 and reached End of Life in December of 2007. If you think about it, PHP 4 is to us what Internet Explorer 6 is to you.
We will be requiring PHP 5.1.6. The easiest way to see if your current host meets the requirements is to download and test using the Server Wizard. If your host does not meet the minimum requirements, open a support ticket and work with them to upgrade your account to PHP 5. In the majority of cases, this should be trivial.