The 4.2 A figure comes from 100 W at 24 V, but 90 W and 120 W seem to be more common ratings for some reason. I have no reports of 5 A at 24 V (120 W) being insufficient for four drivers, so I think 3.75 A at 24 V (90 W) may be enough for three, although I would prefer to have a little more margin (especially given the two motors on the Y driver). There's a fairly large increase in cost if you go over 90 W in a brick-type power supply (the 120 W supply I sell costs almost twice as much as a 90 W supply of similar quality, and it's been hard to find), so I'm guessing that cost drove the choice. I trust Edward and/or someone at Inventables have checked that it actually performed okay with a gShield.
You can probably update the wiki. Here's how I'd put it: at a very minimum, you need a 90 W power supply for four NEMA17 motors driven from a gShield with three stepper motor drivers (with the two Y motors in parallel on one driver). For example, a 24 V 3.75 A supply will work, but the ideal supply would have a little more headroom -- say, 24 V 4.2 A (100 W). You'll need 100-120 W to make the most of a four-driver board, such as the buildlog.net shield or one of its clones -- for example, a 24 V 5 A supply. If you have NEMA23 motors, how much power you need and how much your gShield can get out of them depends on their impedance and rated current. You need 150-200 W for four small (50 mm) NEMA23 motors rated 1.5 to 2 A. With lower impedance motors (such as the 2.8 A motors sold by Inventables as of July 2014), the gShield won't be able to achieve the rated current, so a smaller supply will do (say, 120-150 W). It may be a good idea to connect the Y motors in series in this case, or to use a four-driver shield.