I don't know if there are any books specific to coaching 3 and 4 year olds. I would suggest looking at articles and books about child development. You need to understanding the different stages that children go through. This way when you see them act or preform a certain way you will know what is normal. For example, children at the age like to play near each other, but not necessarily with each other. You might also want to read up on how to deal with parents, especially the pushy ones.
I do suggest that you make the experience fun. Enjoy the children for who they are and don't expect a lot out of them - remember they are just little ones.
Education.com has many great resources that will help you get started in understanding this age group. Here are a few to point you in the right direction.