The most accurate summary of the United States' involvement and military role in World War I from the provided options is:
4. The U.S. became involved in World War I after the Zimmerman Telegram, and sent troops to provide military reinforcements and support.
The Zimmerman Telegram, which was sent by Germany to Mexico proposing a military alliance against the U.S., along with unrestricted submarine warfare, were significant factors prompting U.S. entry into the war. The U.S. declared war on Germany on April 6, 1917, and subsequently sent troops to support the Allied forces.