So I was taking apart a Nexus 7 for the first time today...

Yeah it is definitely not a mic. Its a little metal dome button.
iFixit has some photos of the 4 buttons but no word on what it does.
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I just can't think of what the purpose would be except either a reset button or maybe a camera button had they actually put that rear camera in there.
 
So, I have this old nexus 7 laying around with a broken screen and you know what I pushed the number 4 button and NoraMelua was right. I did gain a million dollars and a random iPad owner was killed. No, no not really, but the money would have been nice. I pushed it and it did nothing, nothing at all, no blue smoke, no shorts, nothing on the screen to show that it sent Google a message saying I pushed something they didn't want me to. I must have pushed it two dozen times and still nothing. You still have to wonder why its there.
 
So, I have this old nexus 7 laying around with a broken screen and you know what I pushed the number 4 button and NoraMelua was right. I did gain a million dollars and a random iPad owner was killed. No, no not really, but the money would have been nice. I pushed it and it did nothing, nothing at all, no blue smoke, no shorts, nothing on the screen to show that it sent Google a message saying I pushed something they didn't want me to. I must have pushed it two dozen times and still nothing. You still have to wonder why its there.

Freakin' knock it off... my garage door keeps opening and closing on my car!!!!!
 
It's too big to be a reset button. I think there were plans for it, but Asus/Google decided to scrap it. What those plans were, we'll never know..
 
We are all talking about N7s for "pedestrians"

You don't naively think that the Pope or **** Cheney are getting the same hardware as we do, do you?

Actually, I may be only half-kidding. I remember reading an article about some electronics, with an ubber-design line for the ultra wealthy, like cell phones costing 25,000 dollars...!
 
LOL, Cheney's first name was automatically censored just because it happens to be a 4-letter word.
 
The mystery KEY4 button on the N7
Well I pressed the mysterious KEY4 on N7 and ... know what it takes?
Nothing. He is not active or Does not activate anything.
The mystery is loose!
 
I know this is kind of late response lol, but I just noticed the button recently and wondered what it was. If you read the print right above it, it says "VOLUME - RESET". I think that's because the one to the left/above it is the volume down then theres the "mystery button". It has to just be an aborted reset button, or maybe it does kill an iPad user... The world will never know :p
 
Hmm yeah, I did it with the Nexus powered on (5.1 lollipop), and it didn't do nothing. I didn't test button 4 yet on the recovery boot screen. But it is an interesting button.