I can't find where there is a picture of one of the machines with 4 spools of thread loaded. I suspect the four pins are to put 4 different colors of thread on. You only load one color through the tension discs and the needle at a time. it's just convenience.
I have a separate thread holder for 8 or 10 spools of thread that I use. It just keeps everything handy. Also, I use a lot of large cones and they won't fit in the smaller side thread holder.
This is the thread holder I use.
https://www.brother-usa.com/products/sa503 I bought it about 10 years ago and the price has doubled. Looks a little bit nicer than mine, but I am happy with what I have.