While this may seem like a small difference in making the final yarn, the structure of the yarns is very different. The 4-ply yarn is very smooth, with the individual strands lying next to each other and the surface reflects more light. A 4-strand cabled yarn is bumpier and has a pixelated effect. The colors of the yarn become dots throughout the yarn, and the reflective quality of the yarn is broken up. Here is a close-up of the yarn texture between 4-ply and 4-strand cable in a knitted sample:
Since I knew I wanted to make a 4-strand cable, I divided my sample into four sections and spun them separately. I spun the first section on my smallest spindle and decided that it would be a waste of time to wind that small amount of yarn on a bobbin and repeat that four times, so I then spun a short section of white wool to act as a divider between the first and second sections of the colored singles. Then I wound that single on a bobbin and spun the third and fourth singles with white wool in the middle to divide them. So I ended up with two bobbins that would allow me to easily ply two singles together:
So if I divided my sections evenly and spun them consistently, I will be able to ply the yarn from the two bobbins together and the white wool in the centers will match up to mark the division between the two 2-ply yarns. I've spun the first of the singles together and was very happy to see that my approach worked very well:
There was very little overlap of colored yarn to white yarn, and now I'll keep going until the bobbins are empty. When I wind the 2-ply yarn back on a bobbin, I'll fill the first bobbin until I reach the white wool. Then I'll break the yarn and wind the next section of colored 2-ply on the second bobbin. Finally, I'll be able to easily ply those two 2-ply yarns together to create the final 4-strand cabled yarn.