Mass of Substance = Volume of Substance x Density of Substance
Density of oxygen (liquid) = 1140.0 kg/m3
Density of oxygen (liquid) in ounce/gallon(UK) = 1.1425575876600793
Mass of a Substance = 1.1425575876600793 x 4
4 gallon(UK) of oxygen (liquid) weighs 4.570230350640317 ounce