I have yet to find a zener that accually drops the voltage off at 4.7v...i have bought 3 brands, one was 3.8, the other 3.9 and the other 4.1..WTF and all where 4.7v zd's!!!
So to who ever buys these, make sure after you put it on you test it. I just used a thing i made to build pressure to the map.