One joule of energy will raise the temperature of 1g of water 1ºC. 1L of water is 1kg =1,000g, so 9,000L will need 9 million joules to go up 1º.
The watt is a joule/second, so 4kW is 4,000 joules/second. So 9 million / 4,000 is 2,250 seconds or 37.5 minutes.
That assumes no loss of heat...