The idea at its simplest is to spin 4 singles with extra twist, make 2 2-plies with extra twist, then cable (ply) the 2 plied yarns together. Here's what I started with:
I spun each color onto a bobbin. From my little experimentations, I knew that the instructions to add extra twist were important. Since it was only half an ounce, it spun up pretty quickly.
Next I plied the plum and chartreuse then the chartreuse and teal. You can get an idea from the ends pictured here. I really can't stress enough that you need lots of energy in the plies. Judith MacKenzie McCuin stresses the need for this extra twist and recommends making 2 passes at the plying. That is, ply it the way you normally would for a balanced yarn then pass it through again to add the extra twist. I didn't do that but would next time.
You can see it better if you click on it for a larger image. I was originally just going to use the 2 greens but found the plum and thought it would be good to have that deeper tone. I wound up with about 92 yards of pretty sturdy worsted weight yarn that I have no idea what I'll do with. I think it might be interesting in a woven item. Might have to pull out the old Bekah rigid heddle loom. Hummmm.... We'll see.