I would check your district's policies to see what kinds of outside testing they accept before I scheduled anything. What are your goals for the kind of information that you want to get out of this? What, if any, outside programs are you considering applying to in addition to school? Those answers will help determine which test(s) you should have done. Another thing to consider is the amount of "headroom" on the test you choose, and the types of strengths you see in your child. Different tests have different "ceilings", and highly gifted children can sometimes end up not getting a good measure of how advanced they might really be if this isn't taken into consideration.
If you are going to have achievement testing done to help with advocacy, I would have it done as close to the beginning of the school year as you can manage and still get the results in time. If you think about it, if your child is already working at a fifth grade level in some subjects at age four, clearly, in a few months, achievement levels can and will change by years.
