Art is a state of being. An artist paints what he (or she) is. - Jackson Pollock.
By all means teach children how to hold a pencil, encourage them to draw circles, straight lines etc to improve dexterity, but don't try to teach art, rather provide them with the opportunity to learn.
From the age of around two children make their mark - be it on paper provided for the purpose, or any other surface that looks to them inviting enough to be improved on. They will scribble their lines, splat and blot their colours and try to tell you what it all means. We adults don't know whether their story made the picture or whether the picture inspired their story - it doesn't matter. What is important it that this is their first teetering steps towards creativity.
Sadly most adults see the world differently. We have fixed ideas about how things should look, and it is all too easy to try to tell the child to begin putting their creations together in a certain way. We think we are helping by telling them to put this line here, that colour there. We give them colouring books that have coloured pictures already there to copy; painting by numbers; tracing books. While this helps such skills as colour matching and drawing shapes, it has the effect of stifling creativity. The child now thinks that unless their drawing of, say a bird, doesn't look like that particular picture, then it must be wrong. There is no room for fantasy, feelings or ideas, no outlet for self-expression.
How to teach art to a four year old? - give him the tools, light the spark and fan the flames.
This is an objective, stand-alone article and is by no means a rebuttal of riverrun's brilliant wu.