California's 4th Congressional District held its top-two primary on June 2, 2026, following redistricting under Proposition 50 that expanded the district to include all or parts of Napa, Sonoma, Sacramento, Sutter, Yolo, Yuba, Lake, Colusa, and Placer counties. Incumbent Democratic Representative Mike Thompson, first elected in 1998 and ranking member of the House Ways and Means tax policy subcommittee, faces a primary challenge from Democrat Eric Jones, a venture capitalist and nonprofit executive, alongside multiple Republican candidates including Ray Riehle, a small business owner. The nonpartisan primary determines the two nominees advancing to the November general election in a district rated Solid Democratic by major forecasters, with outcomes shaped by local voter turnout, candidate fundraising, and positioning on agriculture, tariffs, affordability, and healthcare access.