Part 4 of Profiles in Courage focuses on the first half of the 20th century and America's new role in the world, and the sacrifices and anxieties that accompany this new role. Part 4 describes the careers of two senators, George Norris and Robert Taft, who risk their careers to establish precedents in this new era. 

The first of these men, Senator George Norris, is a Republican from Nebraska. As a senator, he leads a filibuster of an initiative to arm American shipping in the days before America's entry into World War I. Although personally a pacifist, Senator Norris maintains that this measure would erode Congress's sole power to declare war, and unofficially put America on a path to war. Senator Norris is correct. When the president circumvents Congress following his successful filibuster, the country is soon at war. 

Robert A. Taft, the son of President Taft, is a conservative Republican from Ohio. In 1948, during the course of the Nuremberg trials for Nazi war criminals, he protests the use of ex post facto laws to justify their execution, citing the controversial and troubling legal principle of trying individuals for legal actions. For this, Taft is aggressively criticized by members of both parties, and even labeled a Nazi sympathizer.