Welcome!
With regard to grade level, I should note that these are not criterion-referenced instruments, they're norm-referenced instruments, which means that, while they are very good at determining one's relative standing compared to same-age peers, they are not the correct measure for determining grade levels.
If your question has to do with instructional placement, it is generally more effective to do curriculum-based testing, using whatever reading curriculum your school uses. (Some commercial curricula have placement exams; otherwise, you may consider end-of-course or end-of-unit exams taken from the curriculum.)
With regard to extended norms, they do exist for the WPPSI-IV, but not every examiner has them, as they are in a separate resource. I would agree that you will obtain more stable data from a re-test, probably in an other two or three years, using the WISC-V, which should have extended norms by then. (The WISC-IV is, at this point, an outdated test, and should be used only with great caution.)
