user: Question: When was the Hamaoka-4 unit commercial operation date?
Columns: ['Unit', 'Reactor Type', 'Net Capacity', 'Gross Capacity', 'Construction Start', 'Grid Connection', 'Commercial Operation', 'Status']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Hamaoka-1', 'BWR', '515 MW', '540 MW', 'June 10, 1971', 'August 13, 1974', 'March 17, 1976', 'Shutdown January 30, 2009']
SQL:

assistant: SELECT "Commercial Operation" FROM "Table" WHERE "Unit" = 'Hamaoka-4';