When measuring volume, 4 ounces equals 1/2 cup. A full cup contains 8 ounces. Note that standard American units of measurement are not used in every country.
In the United States, all ingredients are typically measured by volume. In other places, liquids are measured by volume, while solids, such as flour and sugar, are weighed. Many of these countries use metric measurements. In this case, 4 ounces or 1/2 cup of liquids equal 125 milliliters. Solids are more complicated to convert. For example, 1/2 cup of flour weighs 60 grams, while the same volume of white sugar weighs 115 grams.