[1] From 2013 to 2023, the district encompassed the Sierras from Truckee to the Sequoia National Forest, as well as a largely suburban area on the edge of the Sacramento Valley in southwestern Placer County.

The 4th district takes in the cities of Santa Rosa, Rohnert Park, Sonoma, Petaluma, and Cotati, and the census-designated places Boyes Hot Spring, El Verano, Penngrove, Sonoma State University, Kenwood, Glen Ellen, Eldridge, Fetters Hot Springs-Agua Caliente, and Temelec.

They are partitioned by Soda Springs Rd, Union Pacific, Alamo Dr, Leisure Town Rd, Hawkins Rd, Bay Area Exxextric, Shilo Rd, Collinsville Rd, and Montezuma Slough.

The 4th district takes in the cities of Vacaville, Dixon, and Rio Vista, and the census-designated places Hartley and Allendale.

[61] They were, in alphabetical order by last name: Regina Bateson; Roza Calderon; Richard Martin; Robert Lawton; Jessica Morse; and Rochelle Wilcox.

Retired Air Force Lieutenant Colonel Charlie Brown, who was the Democratic nominee for this seat in 2006 and 2008, was "seriously considering" running in 2018,[64] but decided in June 2017 against a third campaign.

However, CDP staff refused to accept the forms after it was alleged they closed doors early to prevent the submission.

In the official Certified Candidate List for the primary election,[72] Morse's ballot designation was left blank.

Under the California nonpartisan blanket ("jungle") primary system, only the two candidates with the most votes on June 5, regardless of party, went on to the general election on November 6.