On the WISC-V, the best predictors are the FRI (matrix reasoning and figure weights) and the ancillary QRI (quantitative reasoning index), which consists of Figure Weights (new to the WISC, but has been on the adult version since the previous revision, nearly a decade ago) and Arithmetic. PRI has been the focus of complaints about being a mixed measure (visual spatial and fluid reasoning) for quite a long time; this has finally been addressed in the -V.
On the WISC-IV, I would look, as sallymom said, at matrix reasoning and arithmetic.
Consider also that the nature of math instruction in North America is quite different moving from the primary years, to the intermediate years, to secondary math. Primary (k-3) level math is essentially a language task, as it is presented in our current educational system, consisting of learning the vocabulary of arithmetic. (And actual journaling and verbal explanations, in the Chicago/Everyday Math/constructivist school.) Math instruction begins to transition, increasingly, to concepts and problem solving beginning around 4th grade, until one reaches higher math at the secondary level.
