We tested at 3:5 and the results were very helpful. However I think if you do the WPPSI-IV after age 4 there are more subtests and richer and possibly more instructive feedback?
That said testing was helpful and really helped me get my mind right about what kind of path we were going to take. We are in private school now - if we waited for the public schools it would be 2 more years before he could start kindergarten and he is already reading.
At preschool we accelerated him from the 3s room to the 4s room which bought us some time (and he got to play with the 5s which was what he really liked).
