A torsion correction to the RR 4-form fieldstrength

The shifted quantization condition of the M-theory 4-form G(4) is well-known. The most naive generalization to type-IIA string theory fails, an orientifold fourplane counterexample was found by Hori in [1]. In this note we use D2-brane anomaly cancellation to find the corresponding shifted quantitization condition in IIA. Our analysis is consistent with the known O4-plane tensions if we include a torsion correction to the usual construction of G(4) from C-3, B and G(2). The resulting Bianchi identities enforce that RR fluxes lift to K-theory classes.