Basic 4 ply yarn is yarn consisting of four separate plies twisted together. Traditionally, ply was used as a measure of the thickness of yarn, sometimes corresponding directly to the size of knitting needles. With the abundance of different fibers, ply has become a less reliable indicator of yarn thickness, and so the number of plies is now simply a description of the yarn. 4 ply yarn, then, is simply yarn with four plies twisted together, just as 2 ply yarn is yarn with two plies.
Yarn of this weight is essentially equivalent to medium-weight yarn. This is also sometimes called worsted weight yarn. Descriptions such as these do not always inform knitters of what size needles should be used with the yarn, and so it is important to knit a small amount of the material to test the needle size first. Most yarns come with a suggested needle size, and this is a good place to start.
The tricky thing about plies is that different fibers yield different size plies. This means that a 4 ply yarn can be very thick or very thin. A good way to think about this problem is to consider the difference in size between twisting together four pieces of thread and four pieces of rope. Both would form something that might be loosely considered 4 ply, but the difference in thickness would be enormous.
In actual yarn production, plies are usually made by spinning fibers to create single-strand yarns. Those single strands are then combined by twisting them together, often with a tool such as a spinning machine or a spindle, although the process can be done solely by hand. This twisting makes the resulting yarn stronger and more durable. 4 ply yarn is necessarily stronger than a yarn with fewer plies made from the same materials. These characteristics are important when picking a yarn for a specific kind of knitted item, as different strengths of yarn are suitable for different projects.
Plying can also be used to create yarn with interesting visual effects. In a 4 ply yarn, not all the plies need to be the same material or color. Twisting together different materials can also change the tactile effects of the yarn. Plies themselves do not even need to be the same size, although plies are usually at least similar in size. All these possibilities make manipulation of ply a valuable technique in hand spinning yarn.