Question

What city grew to over 4,000 people by 1840 and marked America’s move west?

Answer 1

Regarding this subject of History some comments:

In the 1840 Americans began their mayor push west of the Mississippi into lands that were largely unsettled except by the indigenous tribes.

Some went in search of land, some were in search  of gold and silver and in the case of mormons in search of religion freedom.

During this time the city that grew to over 4000 people was santa fe.