We wish to find the number of quarts per person, which we realize will be a fractional amount. In the first instance, the fraction is 4/3, or 1 1/3 quart per person. The second group is 3/4, or 0.75 quarts per person. This is less.
An even easier explanation is that 3 friends splitting 4 quarts means each gets a quart of their own, plus a third of the remaining quart. Four friends divvying up three quarts can't even make a whole quart for each. The three have to put a quarter of the quart into a container for the fourth to enjoy equal shares.
