LMI lmivar problem about the relationship matrix variate and decision variate

I have known that a matrix variate L is specified by lmivar function. If L has the structure that the sum of each row is equal to 0, how to specify such L in LMI toolbox? For example, L = [x1+x2, -x1, -x2, 0; 0, x3, -x3, 0; 0, 0, 0, 0; -x4, 0, 0, x4] is matrix variate, where x1, x2, x3, and x4 are decision variates. How to specify L in MATLAB.