Employing a 4D CCSD(T) potential energy surface, initial-state selected reaction cross sections for the complex-forming gas-phase identity S_N2 reaction Cl⁻ + CH₃Cl′ (υ₁, υ₂, υ₃) → ClCH₃ (υ^′₁, υ^′₂, υ^′₃) + Cl′⁻ have been calculated by means of time-independent quantum scattering theory in hyperspherical coordinates. The totally symmetric internal modes of the methyl group (C–H stretching vibration, quantum numbers υ₁ and υ^′₁, and umbrella bending vibration, υ₂ and υ^′₂) and the two C–Cl stretching modes (υ₃ and υ^′₃) are included. The results for pure C–Cl stretching excitation in the reactants are similar to those obtained in earlier 2D calculations. The cooperative effect of C–Cl stretching and umbrella bending modes is even more pronounced for cross sections than for reaction probabilities. The same holds for excitations of the pure internal CH₃ modes; in particular, the ratio of cross sections for reaction with the C–H stretch excited to reaction out of the vibrational ground state is five orders of magnitude larger than the ratio of the corresponding probabilities. This questions the concept of “spectator” modes in reaction dynamics which is valid only for thermal rate constants where the “spectator” modes play a negligible role due to their low population. Transition state theory rate constants fortuitously show good agreement with experiment while the reduced-dimensionality quantum calculations show larger deviations. Possible sources of this discrepancy are discussed in detail. Neglect of reactant CH₃Cl rotation and the related modes in the transition state (doubly degenerate Cl⋯CH₃⋯Cl′ bend and K rotation) yields very good agreement with experiment.