Final answer:
Rudy lost the fourth race intentionally as his rebellion against the oppressive and racist ideology of the Nazi regime, resisting the indoctrination he faced as part of the Hitler Youth.
Step-by-step explanation:
In The Book Thief, Rudy Steiner lost the fourth race at the Hitler Youth Carnival on purpose as a form of silent protest against the Nazi ideology and its oppressive regime. Rudy is depicted as a character who admires African-American Olympic athlete Jesse Owens and is deeply troubled by the racism and hatred promoted by the Nazis, especially against Jews and other marginalized groups. His small act of rebellion by purposefully losing the race is his way of defying the Nazi's expectations and undermining the glorification of Aryan supremacy that was prevalent at the time.
The Hitler Youth was a paramilitary organization that indoctrinated young Germans with Nazi values, including ideals of racial purity and nationalism as expounded by Hitler in Mein Kampf. The expansions and false beliefs such as the "stab-in-the-back" myth were deeply ingrained into the Weimar Republic's society. By losing the race, Rudy resists the pressure to embody the success of the Aryan race that Nazi propaganda so fervently promoted.