Using data collected from 1992 to 1995 with the ALEPH detector at LEP, a measurement of the colour factor ratios C-A/C-F and T-F/C-F and the strong coupling constant <(alpha)over bar (s)> = C-F alpha(s), (M-Z)/(2 pi) has been performed by fitting theoretical predictions simultaneously to the measured differential two-jet rate and angular distributions in four-jet events. The result is found to be in excellent agreement with QCD.C-A/C-F = 2.20 +/- 0.09 (stat) +/- 0.13 (syst),T-F/C-F = 0.29 +/- 0.05 (stat) +/- 0.06 (syst).Fixing C-A/C-F and T-F/C-F to the QCD values permits a determination of alpha(s) (M-Z) and n(f), the number of active flavours. With this measurement the existence of a gluino with mass below 6.3 GeV/c(2) is excluded at 95% confidence level.