The solid solubility of Si in a quaternary Al-4Mg-1Cu-0.5Si (wt%) alloy has been found to be only about 0.061 at% at 520 degrees C, and 0.093 at% at 560 degrees C, less than 1/10 of that in the ternary Al-Mg-Si system. The excess Si in the alloy remains as coarse Mg2Si dispersoids which produce no significant age-hardening effect. Interestingly, a small change of Si content (from 0.061 at.% to 0.093 at.%) in the solid solution of the alloy has been found to be effective in modifying precipitation microstructure by promoting the formation of fine Guinier-Preston-Bagaryasky (GPB) zones and postponing the formation of the S phase during ageing at 200 degrees C. Consequently, the quaternary alloy exhibits an enhanced age-hardening response during earlier-stage ageing. (C) 2017 Elsevier B.V. All rights reserved.