You can take the WPPSI-IV (or any specific IQ test) no more than once every two years. Prepping kids for standardized testing really should consist only of making sure they are well-rested, properly-fed, alert, relaxed, and comfortable. For some children, trying to cram will make them more anxious about the process than necessary. It may also result in invalid results (and this can go either way--up or down). Even if you managed to pump up their scores by test prep, it would not be a realistic picture of who your child really is, and might result in an inappropriate school placement or program, which can be just as damaging if it's too high as if it's too low.
Also, keep in mind that most evaluators are experienced in explaining the testing process to young children, and in putting them at ease. If you go through the school system for testing, it will probably be someone who routinely tests preschoolers.
