Well, I'll assume that we're talking about technical aspects, and not so much the simple fact that the cabinet No.4 (like all AP Tradition of Excellence pieces) is limited to 20 pieces, costs as much as a house, etc. etc.
The Cabinet No. 4 is unique in it's gear train layout. This watch is a chronograph tourbillion with a 10 day power reserve which is achieved by twin mainsprings, and a gear train layout that eliminates the 3rd wheel. This is to lighten the load on gear train torque, which is essential for longevity of the movement with such a high power reserve. The dial is cut out in relief to display the barrels.
Unless your looking for other specifics?
**Edit for spelling mistakes, typos, etc.
BTW, since you're posting again, can we assume you're back?