The if statement must have a scalar logical statement next to it (either true or false) but what you have is a numeric matrix.
If what you are trying to do is extract only rows where x(:,2) > 0 and x(:,3) < 0 and x(:,4) < 0 and x(:,5) < 0 for the regression, but it may be the case that this does not have any data, I would suggest.
rows = x(:,2)>0 & x(:,3)<0 & x(:,4)<0 & x(:,5)<0;
mdl = fitlm(x(rows,:),z(rows));
Really you wouldn't want only one row to satisfy the condition as you would require that x(rows,:) is an invertible matrix.