4 ply, otherwise known as fingering weight yarn, is the product of 4 strands of twisted yarn. It's often used for projects that require detail and is usually super soft.
What is the difference between 4 ply yarn and 8 ply yarn? 4 ply yarn is much finer then 8 ply yarn. Its light, uses needle size 2.25-3.5mm and is commonly used for socks, babywear and lace work.