The 4th District covers a vast and predominantly rural region that includes 12 counties – Butte, Colusa, Del Norte, Glenn, Nevada (partly), Placer (partly), Shasta, Siskiyou, Sutter, Tehema, Trinity and Yuba Counties.
Nielsen is the elected assemblymember from California's 2nd district. The district is one of the largest in terms of coverage area including Butte (7.26%), Colusa, Glenn, Modoc, Shasta, Siskiyou, Sutter, Tehama, and Yolo (6.77%) counties.