Typically in residential wiring cables the ground wire is not included in the conductor count. If you bought other type cable, for almost any other application, the total number of conductors would be included in the count. So yes, 12/3 NMD, NMW, would include a black, red, white plus a bare conductor. If you bought a 12/3 cable in cab tire (the black rubber type flexible stuff) it would include a black, white and green, or three other colours depending on the application it was desigend for and the company making it.
For ground wiring, I like to keep them as continuous as possible. I would probably bring my incoming ground wire, very long from the cable, wrap it around the box screw and then to each receptical in turn with one long, uncut run, all from the cable. In lieu of that, to the box screw, then to a wire nut (connector / Marrette) with two pigtails... one to each receptical ground. If the wire is stranded you may want to crimp a lug on the end, depending on how well the connector is made for that style of conductor.
I am not sure what the point of the four banger receptical would be. I have installed this config in walls a few times and it is a mistake for most applications. Most wall wart AC adapters cover too many recepticals, the six banger receptical splitters cover the other two recepticals and you only get use of two of the wired in ones, the current capacity is still only 15 amps between the four and that limits what you can accidentally use at one time and a few other reasons I can't think of right now. They do work well for light draw test equipment (say electronics).
The number 12 wire may be a good idea if the extension cord has some length to elimate voltage drop and be easier on higher powered equipment (saws, routers etc..) but then you probably wouldn't want to share a bunch of equipment like that on that quad box. Individual circuits are still the best bet for when your grandson comes in and cuts that piece of wood or blows off his dirty pants while you are jointing that piece of briar you just bought.
If you O/C protect the 12/3 cable with a 20 amp circuit then you need 20 amp recepticals (have a T slot in the neutral side) and you are over fusing any portable power tools you use. You may lose some of the human protection afforded by the recommended max circuit capacity for the equipment.
In short, if you are going to open up your walls, spend the time and money to put a few outlets around the room on individual circuits. Possibly one on the ceiling for who-knows-what later. You don't have to use them. I wired mine with one receptical per breaker..kept them high for over workbenches. Any 240v circuits will have to be crawling through the attic or kept close to the breaker panel or piped across the ceiling after.
BTW: There are defined standards for every current and voltage rating of receptical so they can't be used in the wrong application. It is a good idea to stick to these standards. I believe I have found charts on GE or Hubbel websites with pics to identify each type.
Personally, If I were you, I would pay for a wiring inspection. You run all your cables to the boxes and call them to come and then again after the recepticals are installed and closed up. I would run the wire, install the boxes and wire the recepticals and leave hanging for the rough in inspection. (the wall finisher will not like the receptical wired in bu they can be turned sideways and pushed through the holes before mounting the drywall etc... Inform the inspector you are not too sure and have a close look, He will advise some requirements, some tips, and some hints of how to make it right or better, usually. After the first wave of inspection, hook up your breakers in the panel. If nervous about that get some help, friend, passing electrician etc... With inspection, you'll feel better, your home insurance will feel better after a fire, and you just bought yourself some protection against insurance, weasel out.
Josephi - You read my mind. A pair of duplex outlets was what I had in mind by "quad in a box". I honestly did not intend to be vague.
I had a question concerning the ground wires in wiring a quad box in a branch circuit (both outlets to be run in series from the same hot). Resources I have found have been vague. My understanding includes that a wire attached to the ground terminal of the first duplex outlet would be pigtailed with the upstream ground wire and a wire which is screwed to the box, and that the wire attached to the ground terminal of the second duplex outlet would be pigtailed with the downstream ground wire and a wire which is screwed to the (metal) box. So the box would contain exactly 2 connectors and two wires would be screwed to the box, possibly at the same place. Does this seem like the best way to you? I can think of equivalent configurations, but this one seems good. Another possibility seems to be to use a 3rd pigtail connecting the first two pigtail connections and connect them to the box that way instead. Which way seems preferable to you?
BTW, using 12-3 cable for my run, every wire I mentioned connecting in the paragraph above would be bare (right?).
Thank you! Bill