Final answer:
The statement that union membership reached its peak around 1930 is correct, correlating with the National Labor-Management Relations Act of 1935 and strong government support, but union membership declined after the Taft-Hartley Act of 1947.
Step-by-step explanation:
Based on the historical account of union membership in the United States, statement 4 is supported by the information provided: Union membership reached its peak around 1930. This followed the passage of the National Labor-Management Relations Act of 1935 and the strong encouragement of the U.S. government to form unions during the early 1940s, aligning with war production efforts. However, the passage of the Taft-Hartley Act of 1947 gave states the power to allow workers to opt out of unions, leading to a decline in union membership after World War II.