Most do, yes; if the monitor has been made in the last few years, almost certainly. It's called the VESA standard, put in place so mounting wouldn't be such a hassle when buying mounting hardware. That said, some monitors have them hidden or behind an enclosed plastic panel (for aesthetics reasons). I, obviously, cannot speak for all makes but my two Acers required I forcibly remove the plastic panel on the rear to access the four holes. I'd suggest you look up your monitor specs and look specifically for a VESA pattern somewhere in the description. A place to start: http://goo.gl/yCAvK
I'm not sure. The one I purchased has them which gives you the flexibility to move the monitors horizontal or vertical. Not to mention to keep the monitors sturdy. This was a really good purchase and have not had any issues.
Keep in mind, larger monitors usually have six holes, not four. The idea is that you use the middle set plus the ones on the right OR the ones on the left to better align the monitor according to your set up. Don't let the six holes throw you though.