dx1/dt=-k31*x1+k13*x3+u1;
dx2/dt=-k42*x2+k24*x4;
dx3/dt=k31*x1-(k03+k13+k43)*x3;
dx4/dt=k42*x2+k43*x3-(k04+k24)*x4;
y1=x1;
y2=x2
