“…Together with Bose symmetry, they allow to reduce the form of the soft anomalous dimension matrix to a relatively simple parametrisation in terms of a few scalar functions. Of particular interest is the appearance, at four loops, of contributions proportional to quartic Casimir operators of the gauge algebra, which are fully known for the simple case of two Wilson lines [61,62,79,80], but, as yet, undetermined in the general case, although partial results have begun to emerge [53,81]. Our results provide all the necessary colour ingredients for the complete four-loop calculation, and other tools are available for the study of colour structures at high orders, such as the effective vertex analysis of ref.…”