From the law on plurality of wives follows the law on equality of treatment. Mohammed, who allows for four, would have everything equal among them : food, clothing, conjugal duty. This law is also established in the Maldives, where a man may marry three wives. 
The law of Moses even requires that when someone has married his son to a slave, and later he marries a free woman, he should diminish none of her clothing, food, and duties.  More could be given to the new wife, but the first wife must not have less.