That's the story with many hosts. PHP4 may be a "stable" product, but server admins have to weigh the needs of the server and the server community with the demands for the newest technologies. Its a VERY delicate balance to port over to PHP4 from PHP3, and the act to get them to run side by side during a transition is admirable indeed. But when it comes down to it, most PHP3 scripts can fail miserably under PHP4, and it more or less saves a headache to not upgrade for the time being until all your tests are complete.
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Shalazar
http://www.charisma-carpenter.com