Abstract Acoustic emission has been measured during the deformation and fracture of two quaternary Al-5.5wt.%Zn-2.5wt.%Mg-1.6wt.%Cu alloys with different grain sizes as a function of aging. The results have been compared with a previous study of the ternary alloy (Part I). It has been found that in the larger-grain-size alloy the most energetic acoustic emission source is the formation of intense slip bands in the peak-aged condition similar to the ternary alloy. However, reducing the grain size (by the addition of zirconium to the quaternary alloy) suppressed both this emission source and weaker sources in overaged states. The effects are interpreted in terms of a reduction in the area of slip events. S phase inclusion fracture was observed to have occurred in the quaternary alloys and may have generated small-amplitude emissions in peak-aged material.