Answer:
impose a tax on imported widgets - if the government imposes a tax on imported widgets, imported widgets will become more expensive to consumeres, making consumers flock to domestically produced widgets, prompting domestic firms to increase domestic supply to at least 8,000 widgets.
impose an import quota - the government can also simply impose an import quota of 4,000 widgets, which will oblige consumers to buy at least 8,000 domestic widgets if they want to satisfy their demand of 12,000 widgets.