California's 4th congressional district is a U.S. congressional district in California. The district is located in the northwestern part of the state, and includes all of Lake County and Napa County, most of Yolo County, and parts of Solano County and Sonoma County. Major cities in the district include Davis, Woodland, Napa, Vacaville, and most of Santa Rosa. The new 4th district is solidly Democratic, and is represented by Mike Thompson.

Lake County is a county located in the north central portion of the U.S. state of California. As of the 2020 census, the population was 68,163. The county seat is Lakeport. The county takes its name from Clear Lake, the dominant geographic feature in the county and the largest non-extinct natural lake wholly within California.