Converting 400 grams into standard ounces provides 14.11 ounces, each ounce is approximately 28.35 grams. Grams are a unit of measure for weight, while cups are a measurement of volume, which prevents conversion.
It is possible to convert liters into cups for an accurate volumetric measurement. Volume can also be represented by the cubic area of a container, while mass is a function of the density of the substance inside the same container. This is why volumetric measurements are commonly used for measuring liquids and gases while solids are commonly measured based on their mass or weight.
Further, density can be determined by finding both the mass and the volume of an object through the use of displacement. For example, 400 grams of gold would have significantly greater density than 400 grams of cotton, as gold will displace less volume compared to the cotton.
This is how astronomers determine the density of stars and planetary bodies at a distance, commonly relying on nearby points of reference to determine volumes. A good example of this is comparing Jupiter and Earth based on their density. Jupiter is much larger, but is predominantly gaseous in nature while Earth is much smaller and composed primarily of rock and water, substances with greater density and weight.