[eluser]Pascal Kriete[/eluser]
While the underlying framework is written with PHP 4 in mind, it won't affect the way any of your own code is interpreted. So the easy answer is: Yes, it fully supports it.
There is one case where you might say it doesn't. Extending libraries. You won't be able to call parent::__construct(), because that function doesn't exist. That's not really a big deal though.