The equations to be solved for x1, x2, x3 and x4 are
2x1 + x3 + 3x4 = 0 ...(1)
2x2 - x1 - x3 - 2x4 = 2 ...(2)
x1 - x2 + x4 = 2 ...(3)
To find a unique solution the number of independent equations must be equal to the number of variable. In the problem, there are 4 variables and only 3 independent equations that have the variables.
It is not possible to find a unique solution for the variables.