Absolute differential cross sections for the 3p 'S0~3p'('P, /2)4s, ('P3/2)4s excitation in argon have been measured at impact energies of 400 and 500 eV, and at scattering angles between 1. 75' and 10. 3' by electron-energy-loss spectroscopy. The integrated cross sections and generalized oscillator strengths (GOS's) have also been determined at 400 and 500 eV. The optical oscillator strengths have been determined, by extrapolating the GOS's to zero momentum transfer, as 0.222+0'03 for the ( Pl/2)4s state and 0.058+0008 for the ('P3/2)4s state. These data are compared with the results of other optical measurements in the vacuum-ultraviolet region. The errors are estimated to be less than 14%.