4. Charles is a self-employed artist who maintains a qualifying office in his home. Charles has $40,000 gross income from his business and incurs $28,000 in supplies and other business-related expenses. Charles's mortgage interest and real estate taxes allocable to the office total $6,000. Other allocable expenses total $8,000 and consist of depreciation, utilities, insurance, and maintenance. What is Charles' total home office expense deduction?