Today there is a large set of coloring books. You can find coloring-cursive books, books outliners, for boys (all sorts of appliances, machines, robots, etc.), for girls (fairy-tale characters, dolls, funny funny little animals). There's even a wallpaper coloring books that opens a wide field to have enough space for young artist.
In 3-4 year old kids are more confident in coloring. They're not trying to draw freely, and act within the circuit, trying to repeat the figure bends.