I synced my carbs on my gpz with a homemade apparatus two carbs at a time. I was pretty easy and I cross checked all four to be certain. I also gauged the slides to make sure the gap under them was the same. I should have kept that bike.
Interesting... I thought a GPz had constant velocity carbs on all models. The openings on the throttle butterflies would be what is used as a gauge of sync and not the vacuum pistons. They will all sit there like a bump on a log until the bike is running and throttle twisted and I doubt it would be possible to measure how far open they are.
Doing two carbs at a time is possible with CV type carbs. You would first sync 1&2 to each other then 3&4 to each other and then connect your double gauge to 2&3 and adjust the center sync adjustment and all four would be then synchronized.
Texas Hill Country