Please explain "The Big Four" in the United States of the 1860s.

The Big Four were the founders of the Central Pacific Railroad in the 1860s in Sacramento, California. 

"Ambitious men like Leland Stanford, Collis P. Huntington, Mark Hopkins, and Charles Crocker - the financial backbone of California's Central Pacific Railroad and known to history as the Big Four."  

Collis P. Huntington, Mark Hopkins, Leland Stanford, and Charles Crocker who were responsible for the greatest historical event in transportation history.

"The Big Four conceived this enterprise and brought it to a successful ending after years of daily struggle that would have exhausted the patience and spirit of ordinary men. Huntington looked after the financing of the company. Crocker, with his tremendous energy, forced the construction of rails over the snow-crested Sierra and across the burning deserts of Nevada and Utah. Stanford kept his energies on the main points leading  to success, and Hopkins saw that none of the money was wasted. That pioneer railroad line of the middle ’60s formed the basis of the gigantic Southern Pacific system."

For biographies on the Big Four individual men, click on the first link, for more information about the railroad, click on the second link. The third link provides additional information about the Big four.