Lake No. 4 or Polygon Lake, about 1.8 hectares in area, lies 1000 metres west-northwest of Ironmask Lake (092INE073) in the same general depression, although its surface is 9 to 12 metres lower. The divide between the two is only about 3 metres above the surface of Ironmask Lake. Diorite of the Cherry Creek unit of the Late Triassic-Early Jurassic Iron Mask batholith outcrops along the south shore. Polygon Lake is within the boundaries of Crown grant Lot 4667 (Winty) which is part of the Crescent property (092INE026).

The lake is roughly circular and the shorelines steep. In October 1937, at least 1.5 metres of brine was present. The shoreline was encrusted with dried salts but no permanent crystals were noted. A sample of the brine analysed total solids 11.67 per cent with a specific gravity of 1.1075 at 16 degrees Celsius. The composition of the solids were 32.8 per cent Na2SO4, 62.0 per cent MgSO4, 1.6 per cent NaCl, 1.4 per cent Na2CO3 and 2.1 per cent CaSO4. The similarity of the brine to that of Ironmask Lake is notable. When examined, the lake would have contained 635 tonnes of mixed salts per 0.3 metre depth, or a total probably in excess of 2721 tonnes (Bulletin 4). The sodium sulphate mineral assumed to be present is mirabilite and the magnesium sulphate mineral, epsomite.