Last time I checked, 40 degrees Celsius was 104 degrees Fahrenheit. 4 degrees Celsius is close to 40 degrees Fahrenheit (39.2). Is that what you meant? As far as twice as cold goes, it could be the same as half as warm. In an absolute sense zero degrees Celsius is 273 degrees above absolute zero, so half as warm would be 136.5 degrees above absolute zero, or -136.5 degrees Celsius. Or if twice as cold means that it feels twice as cold, you could base it on the body's perception of cold, whatever that is.