The common difference if there is one is the constant difference that occurs between any term and the term before it.... in this case:
There is no common difference,
dx=18,20,16,18 the difference or velocity is not constant...
d2x=2, -4,2 the acceleration is not constant...
d3x=-6,6 the thrust is not constant
Now we might be tempted to say that:
d4x=12 and say that that is constant and we COULD make a quartic equation fit all the data points, but without further data points in the sequence there is no mathematical proof that the quartic equation would produce accurate data points outside of the range given...
And solving a system of five equations for five unknowns is tedious for such a problem...a^4+bx^3+cx^2+dx+e=y