WordPress have announced that once wordpress v3.2 is released, they will no longer support php 4 or mysql 4. This announcement has been expected for some time. The last time that a php 4 update was relased was in August 2008 and mysql 4 received a final relase a few months later in December 2008.
If your host is still using php 4 or mysql 4, you should contact them and insist that they upgrade to php 5 and mysql 5. php 4 and mysql 4 have not been updated in the last 19 months!
As wordpress will no longer be supporting php 4 or mysql 4, we will also not be supporting php 4 and mysql 4. This is not just for the same reasons as wordpress, but also because we believe they will specifically be using php 5 which will cause wordpress to no longer work on servers running php 4.