A new person to crochet recently asked me what the 4 ply yarn I stated I used in my pattern meant and if it had anything to do with gauge.
4 ply is the weight of the yarn or thickness and is also called medium . It really means 4 strands of thinner yarn have been twisted together to produce that yarn. You find the weight of the yarn usually just above the name of the yarn something like: Worsted - Medium - 4. Worsted means several strands of thinner yarn has been twisted together to make up the yarn. Medium is the over all thickness you'll achieve in your finished piece and 4 is the the number of worsted yarns
4 ply is a weight more commonly used in crochet. One of the easiest to find brands of 4 ply yarn is Red Heart. Usually Wal-mart, Joanns, Michaels, etc have a large variety of colors . I always like to list the weight or type of yarn I used to make the item from the pattern so if it is a special type of yarn you know that up front.
There are of course thinner and thicker yarns availible. If you substitute a thinner yarn like a sport weight--3 ply-- you'll end up with a smaller finished piece than what the pattern states. Like wise a thicker yarn will get you a larger finished piece, but I really don't recommend switching as not all patterns can tolerate the change and you may lose some of the details that attracted you to that pattern in the first place.