I think flash cards are a great way to teach any language (along with other methods), especially to a complete beginner. I would print them from the net or make them yourself. The ones you buy are quite expensive and you can do a good (if not better) job yourself. If you do them yourself, you can also target the words to ones that the 4-year-old will use most often to begin with, before moving on to a wider vocabulary.
Student or Learner