The structure of the WJ and WISC are significantly different, though they do usually generate comparable results, so the drop from WJ GIA to WISC FSIQ isn't necessarily the biggest deal. I would not actually describe her WISC-IV GAI as considerably lower than her WJIII GIA. It's only 8 points, which, when one considers the age at first administration, regression to the mean, and change from one test to another test, is not that substantial.
Did you actually have the WISC-IV done, or the WISC-V? The -V is generally quicker to administer, especially if only the subtests required for an FSIQ (and GAI) are given. Under an hour is not at all unusual for the -V, even for a bright kid.
