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To start with my conclusion, there are plenty of flat keys in Suzuki books before volume 5, so I wondered what the teaching moment of the Concerto in G Minor could be. I watched the CadenzaStringsNC guy play the piece. My guess is that this piece is "held back" until book 5 because it looks like it involves shifting competence into a few higher positions.
I looked back at my Suzuki book 1 and realized that it only introduced a few # keys.
Book 2 gingerly introduces F and Bb Major.
For the record, here you can find the contents of the Suzuki books 3, 4 and 5 to see which pieces (and in which keys) are included:
Book 3: http://www.amazon.com/Suzuki-V.....0739048155
(includes: Gavotte in G Minor (Bach))
Book 4: http://www.amazon.com/Suzuki-V.....0874871506
(includes pieces in A and D Minor)
Book 5: http://www.amazon.com/Suzuki-V.....0874871522
(Concerto in G Minor, Op. XII, No. 1, F.I.N. 211 (Allegro, Adagio, Allegro)(Vivaldi)
That's my guess.