  The soft anomalous dimension governs the infrared divergences of scattering
amplitudes in general kinematics to all orders in perturbation theory. By
comparing the recent Regge-limit results for $2\to2$ scattering (through
Next-to-Next-to-Leading Logarithms) in full colour to a general form for the
soft anomalous dimension at four loops we derive powerful constraints on its
kinematic dependence, opening the way for a bootstrap-based determination.
