Looking at the differences between the terms, we have a set of negative increments: -10, -4, -4. So we would expect this to continue with -10, because of symmetry, making the next term 0-10=-10.
With only four terms in the given series, we would expect to find a third degree polynomial that will provide the series. When we do this we get -x³+9x²-30x+40 as the series generator. The coefficients are found by substituting values of x from 1 to 4 and solving a system of four equations for the four coefficients.
The series is then 18, 8, 4, 0, -10, -32, ...