If the temperature decreases 2°C per hour, and 4 hours went by (from 1:00 am to 5:00 am), then you simply multiply 2 (the rate) by 4 (the time). This would bring you to 8. After this, you subtract 8 from 4 (because 4°C was the original temperature) which should be -4 (negative four). Your answer would be -4°C at 5:00 am. I really hope I am right; however, I am truly sorry if I am not.
2 × 4= 8
4 - 8 = -4