Good trick,
However you do need to pick someone who was married in the last century or the trick wont work. Also some people actually can't remembr what year they were married.
I prefer to borrow just one ring, get four spectators to choose a number each and set the combination to those numbers, but of course it won't work. Ask the spectator to set the combination to their birth year and Viola!
At $75 it sounds expensive but it's well made (you can even use it as an ordinary padlock) and will last you a life time.