It took a whole day to get the right answer for you.
(I had to do a Vulcan mind meld with my code-writing engineer and speak geek...ouch)
The technical specs as I understand them are-
The previous 3.0 circuitboard was spitting out data as fast as it could go @ one and a half million instructions per second. The new Z4 series will be processing @ sixty million instructions per second. The Z4 is also performing additional digital signal processing duties including input learn and noise floor scaling, cc assignable inputs, individual trigger channels, and sending info out in a much faster and cleaner way with MIDI stream packing from a state of the art processor. (None of these more complex features were possible to write on the 3.0 platform)
The Triggers themselves are not changed, but the resolution of the trigger strikes are calculated much faster and with much greater accuracy, resulting in much more precision thoughout.
It has been created with the latest software and hardware compatibility in mind so that Stylus, Ableton and other programs can "see" the MIDI notes that were over too quickly before to be recognized by that particular software. This is a very helpful solution for a lot of Zendrummers who were adding MIDIox as a workaround to extend the notes or found the Zendrum incompatible with a certain program they already owned.
Regarding shipping back for retrofit work after June 1-
After we have communicated privately about your retrofit details and created a work order together with specifics about any/all additional modifications, I will ask you to ship Domestic FedEx Ground with the return agreed to via same service and include the return shipping charges in your work order total once it has been calculated.
Ship in a double thick box carton if possible inside the gigbag with lots of bubble wrap packed around it. Insure it and specify signature required upon delivery.
We've got until June 1 to clarify any retrofit modifications and schedule shipping and benchtime into a satisfactory turnaround estimate.