Dear Svetlana,
unfortunately there's nothing you can do on your user side as only hosting provider can change PHP version. In this case to upgrade from PHP4 to PHP5. I had the same issue - many Open Source PHP applications require PHP 5 to work properly, I sent multiple e-mails to Yahoo! hosting service representative regarding when they plan to upgrade to PHP5 or to offer both PHP4 and PHP5 (some hosting providers do that) but didn't receive any feedback.
I would advise you to choose middle sized hosting provider as they offer better customer service and have the most recent stable PHP, MySQL etc. versions.
Take care,
Maris