Cosmicflows-4 and the Scale of Cosmic Topology: A Null Test for Dodecahedral Anisotropy in Local H₀

The Poincaré dodecahedral space (PDS) model predicts directional modulation of the Hubble constant associated with the dodecahedral face geometry. Using the Cosmicflows-4 grouped catalog (38,053 groups), the local H₀ field is decomposed into spherical harmonics up to ℓ = 20 after dipole subtraction and galactic plane masking (|b| > 20°). No statistically significant signal is found at ℓ = 12 (the dodecahedral harmonic; p = 0.35). A significant excess at ℓ = 5 (p < 0.005, > 4σ) is detected, but its direction (l, b) ≈ (305°, −33°) and dominant azimuthal mode (m = 1) identify it with the Shapley–Great Attractor axis rather than with pentagonal face symmetry. This null result is not merely compatible with the PDS model - it is predicted by it: the Cosmicflows-4 catalog spans ~200 Mpc, approximately 8% of the predicted fundamental cell size L_cell ≈ 2300 Mpc, a scale at which local gravitational structure dominates over topological signatures.