Whenever looking at sequences, you have to attempt to find a common factor between the members of that sequence, be it the difference between them, a common divisor, a common multiplier, etc. In this case, we can see that if we divide each member by the previous one, we get a the same number:
100/-20 = -5
-20/4 = -5
So our sequence, starting at 4 is represented by the following recursive equation:
