I think what John may have been saying is there's an error in the Haynes manual that calls out an incorrect procedure. Since I don't have that manual, that's just a guess. However, the #4 carb is the 'baseline' carb that has no adjusting screw, so you adjust the other 3 to it. Whether you do that in pairs with a 2-channel tool, or all together with a 4-channel tool doesn't make much difference as long as you double-check it after all adjustments are done, because tweaking one carb sometimes has an effect on one of the others. With 4-channels you see that effect immediately, with a 2-channel tool you wouldn't notice it until you went back to re-check later. So, using a 4-channel tool is a lot easier and faster.