Basically, whenever the subtest score is 17 or higher, it's considered a ceiling on that particular subtest. I think you can take a WISC IV one year after you take the WPPSI (especially since it's a different test so there should be virtually no identical questions with the preschool version). Your son is without any doubt gifted, however, WISC IV at some point later is probably a good idea as it is a little longer in length than the preschool test and includes Working Memory. Also, it's probably a bit harder as well as it's designed for older kids. You will get slightly more reliable and complete data than with the WPPSI.
