The temperature at day today = 62 degrees 
Since at night the temperature will fall by 40 degrees, the temperature at night = 62 - 40 = 22 degrees 
By 4 AM, the temperature will again fall by 40 degrees, so the final temperature at 4 Am = 22 - 40 = - 18 degrees 
We can now conclude that there are two successive falls of 40 degrees. 
Therefore, the temperature at 4 AM will be -18 degrees (negative)