A certain linear circuit has four input voltages and one output voltage vO. The following table lists the output for different values of the four inputs. Find the input-output relationship for the circuit. Specifically, find an expression for vO in terms of the four input voltages.

vS1(V) vS2(V) vS3(V) vS4(V) vO(V)
2 4 4 1 20 1 2 2 1.5 4 1 4 2 2 1 0 5 3 1 3.