Symanzik improvement of lattice QCD with four flavors of Wilson quarks

We have determined the non-perturbative O(a)-improvement coefficient csw for four flavors of Wilson quarks with the plaquette gauge action in a range of β⩾5.0. The data are fitted with several Padé approximation formulae to get an impression of the stability. A small extrapolation below β=5.0 seems acceptable.