I read a FABULOUS book when my first (also 4 now) was first born called "The Wonder Weeks." (I buy it for every pregnant person I know!) It addresses just what you are talking about for growth spurts in the first year of life. It specifically is talking about intellectual changes, but I've experienced and heard from others the same to be true for physical changes and growth (learning to walk, etc.). The basic premise is that when children are learning something new, it is very unfamiliar to them and you will see them behave differently. Some may withdraw, some may act out. Sleep patterns might change. And on and on. While this book was about the first year, I think it is 100% applicable at any stage of a child's development. Depending on what is changing, the period of strangeness may vary in length, but what you describes sounds like exactly what this book talks about. It's often very challenging for the parents, but watch to see what he is working hard on and what changes as those phases pass. It is so amazing to watch those little minds and bodies grow and change!