A 12" diameter hole would be 113 square inches. A 4" diameter hole is about 12 square inches. So you would never need a 12" intake hole for a 4" fan/exhaust, as that's TEN TIMES the exhaust area.

Personally, I would use a 6" inline centrifugal fan, and then dial down the fan speed to keep the noise level lower. A 6" fan spinning slowly pushes/pulls as much air as a 4" fan spinning at full speed, but it's not as noisy. Another benefit to that is if you actually end up needing more fan power, well, you've got it right there, just dial the speed back up as needed.

I'd suggest using some lightproof darkroom vents for your intakes. Doran makes an 8" x 8" darkroom vent that is affordable (~$25-ish). I needed two of these to provide enough intake flow for a 6" fan. I also use standard furnace filters on the intake vents, to keep dust/hair/crap out of the cabinet, which reduces airflow and probably explains why one darkroom vent was not enough. There are other intake solutions (elbows and PVC pipe for example), but those Doran vents work great when mounted on the side of the cabinet, look professional, don't take up any precious floor space, and they really are completely lightproof.

For the electrical cord, I hole-sawed a hole in the bottom of the back of the cabinet, then used Great Stuff to seal around the cord, and since the cord's in the back it's not visible, so that's reasonably stealthy. If someone's pulling my (250-pound) cabinet away from the wall, any thoughts of stealth are probably shot anyway.

Unless you're selling a significant amount, I wouldn't bother with CO2. Sure, if you use a cooltube with separate ventilation systems for the light and the rest of the cabinet, and keep the CO2 levels up, you can get a boost at harvest time. But it adds to the expense and complexity. If you're growing for yourself, you won't need CO2, unless you're just chain-smoking stogies. And even if you ARE selling everything you harvest, I don't know that the extra cost/complexity is worth it, but that's really your call. The co2boost thing looks interesting, and simple, but that's another $100/crop. Definitely not worth the cost unless you're selling, and I'd want to hear from someone I trust as to how much it boosts harvest weight. 25 percent? 10 percent? 1 percent?