Circular Spot on Back of Toddler’s Head

My 4 year old has developed an almost circular spot on the back portion of his head. It seems almost bald. What is it?

There are many causes of such balding, including genetic factors such as alopecia areata (or variations of this), fungal infections, etc. You need to take your 4 year old to see a dermatologist specializing in children and/or hair loss.