Amazingly, Zend, "The PHP Company", will sell you Zend Studio, but they will not package a 5.4 version of the Studio Debugger in the related download. I would think they could get an updated version out since 5.4 has been released for over a year...
In any event, a work around has been making the rounds on the Internets.
Here is a full set of instructions for a Debian based system.
It was based largely on this Zend Forum post... http://forums.zend.com/viewtopic.php?t=47568&f=59#p148893
The gist is to lift a 5.4 debugger file from an old Zend Server download an use it instead.
Many thanks to the original posters.