XXXX (pronounced fourex) is a brand of Australian beer brewed in Milton, Brisbane by Queensland brewers, Castlemaine Perkins. It enjoys wide popularity in the state of Queensland, where it is commonly found on-tap in pubs and bars.
The XXXX brand was first introduced in 1924 and is a throwback to the long standing tradition of using Xs to indicate the strength of an ale. The brand name also built upon XXX Sparkling Ale introduced in 1878. The brand is used on the XXXX Bitter, XXXX Gold, XXXX Light Bitter (Lager),XXXX DL (Diet Lager) and Sovereign (Lager).
Previously sold beers, currently not available, include Thirsty Dog (Wheat Beer) XXXX Draught/Original Draught, and XXX Sparkling Ale which is only sold at the XXXX alehouse next to the brewery.
Info gleaned from Wikipedia