18,000 yuan. The unit 'centi' means one thousandth of the interest rate in China's financial industry. 4% is 0.04 thousandths. According to the interest calculation formula, the interest is equal to the loan amount multiplied by the interest rate. It can be obtained that the loan amount is 450,000 and the interest rate is 0.04 thousandths. The calculation result is: one year's interest is 18,000 yuan.