When the Suzuki books were being revised some years back, there was a lot of discussion (? argument?) about this. Our copy of book 4 came with an alternative version of the 1st mvt of Vivaldi A minor, which was trialed across European Suzuki teachers for a few years before everyone quietly went back to the old version (my impression, I don't know the details). If I was asked to discuss it with someone keen on authentic historical performance practice, I'd be citing pedagogical reasons. Which reminds me, a 13 yr old student of mine learning it at the moment would be ready to learn about such differences. The two 8 yr olds would not ...
Bach double 1st violin part goes into 5th position whereas the 2nd violin only needs 2nd and 3rd; book 4 was originally the first place in the Suzuki repertoire to require shifting (lots of alternative fingerings in now from late book 2 onwards), and 5th position isn't encountered till book 5.
Indeed the progression in Suzuki books is conceptually different to exam grades, some pieces introduce "big" new techniques, others will then consolidate or serve to develop something more straightforward.