In theory WPPSI and WISC scores should be directly comparable and across any version, in reality it's probably more true that WISC and WPPSI versions that come out at the same time should be directly comparable but two versions of the same test taken by the same child on the same day could produce quite different results (ie yes the WPPSI iv would likely score lower than the WPPSI III).
Also, with the same test over time it seems likely that a 5 yr old doing any version of the WPPSI in it's last year before a new test is introduced will score higher than a similar 5yr old tested back in it's first year of use.
Although possibly my observations of patterns apply a more to gifted children and less to average children? Disclaimer: not an expert!
