four variables
Note to the Nature of Wealth by John Ruskin

The two first of these variables are included in the x, and the two last in the y, of the formula given at p. 162 of "Unto This Last", and the four are the radical conditions which regulate the price of things on first production; in their price in exchange, the third and fourth of these divide each into two others, forming the Four which are stated at p. 186 of "Unto This Last".