Answer:
The four free states west of the Mississippi River were Iowa, California, Minnesota and Oregon.
Explanation:
The Northwest Territories of the United States was the first region in the United States to completely abandon slavery. The Northwest Ordinance passed shortly before the adoption of the US Constitution (1787) stipulated that slavery and religious discrimination were prohibited in the Northwest Territories.
The states that were later established in the region, Ohio, Indiana, Illinois, Michigan, Wisconsin, and Minnesota, were mostly settled by New England immigrants and retired veterans of the American Revolution. These states prohibited slavery since its creation.
With the creation of the states of California and Oregon in the west coast, there was also agreed that these states would become free states, to keep the balance between slave and free states in Congress.
