1. The Ludlow Massacre occurred on June 4, 1914, when Colorado National Guardsmen attacked striking coal miners and their families at a tent colony near Ludlow, Colorado. The violent conflict resulted in the deaths of 21 people, including women and children.

2. On June 4, 1933, the first drive-in movie theater opened in Denver, Colorado. This innovation allowed automobile-bound viewers to watch movies outdoors while parked in their cars, and quickly became popular throughout the United States.

3. The Eisenhower Tunnel, the highest vehicular tunnel in the world, opened on June 4, 1973. Located in Colorado's Rocky Mountains, the tunnel stretches beneath the Continental Divide and provides a vital transportation link between Denver and western Colorado.

4. June 4, 1975, marked the first official "Bike to Work Day" in the city of Boulder, Colorado. This annual event has since spread to other cities and states throughout the United States, promoting the benefits of cycling for personal health and environmental sustainability.

5. On June 4, 1998, Denver became the first major U.S. city to legalize the use and possession of small amounts of marijuana for medical purposes. This landmark decision has since paved the way for a wider acceptance of cannabis for both medical and recreational use in Colorado and other states.