We get this question weekly and it's not a point and shoot response. So we thought others might like to know the answer, too. It all goes back to our mothers'/grandmothers' day when acrylic yarn was overwhelmingly the most common yarn. A 4-ply yarn referred to a worsted weight acrylic that could be made into sweaters & afghans using a size H hook or size 8 knitting needles. It was usually an inexpensive yarn, the most common being Red Heart 4-ply. These can be found today and are sometimes still called that way.
But 4-ply is really the number of threads that have been spun individually and then plied together. They could be of any weight, so it really doesn't represent the yarn size or quality. Also, lots and lots of worsted weight yarns today are not 4-ply, but may be made up of a single ply all the way up to 12 or more plies. For a knitter or crocheter, finding a yarn that has 4-plies is not only not useful, but most often not necessary for the project.
So the Craft Yarn Council came up with a system of more clearly identifying yarn weights based on thickness and gauge, irregardless of the yarn's construction or number of plies. It is called the "Standard Yarn Weight System" and can be found at www.yarnstandards.com
It's useful to get familiar with it as that is the easiest way to help you locate the size yarn you want for your project. We have signs in the shop identifying the yarns according to this system, if you ever get confused. And, of course, we can help!