Incumbent Democrat Mike Thompson led California's 4th Congressional District nonpartisan primary on June 2, 2026, with roughly 37 percent of the vote, ahead of Republican Raymond Riehle at 24 percent and fellow Democrat Eric Jones at 21 percent. Under the state's top-two primary system, these results position Thompson and Riehle to advance to the November 3 general election regardless of party. The district, which includes parts of Napa, Solano, and Yolo counties, remains rated as strongly Democratic-leaning for the fall contest. No major last-minute developments altered the field before voting, and certification of final primary tallies will confirm the exact matchup.