{ diff(x1, t) = (1*k13*k1*x1 + (-1)*k13*k2*x2*x1/(x4 + k3))/k13,
  diff(x2, t) = (1*k13*k4*x1*x2/(x4 + k3) + 1*k13*k5*x3*x2/(x4 + k3) + (-1)*k13*k7*x2*x1 + (-1)*k13*k8*x2)/k13,
  diff(x3, t) = (1*k13*k10 + (-1)*k13*k9*x3 + (-1)*k13*k11*x2*x3)/k13,
  diff(x4, t) = (1*k13*k6 + (-1)*k13*k12*x4)/k13 }