{ diff(x1, t) = ((-1)*k1*k2*x1 + (-1)*k1*(k3*x2*x1 - k4*x4))/k1,
  diff(x2, t) = (1*k1*k2*x1 + (-1)*k1*(k3*x2*x1 - k4*x4) + 2*k1*k5*x4)/k1,
  diff(x3, t) = (1*k1*k2*x1 + 1*k1*k5*x4)/k1,
  diff(x4, t) = (1*k1*(k3*x2*x1 - k4*x4) + (-1)*k1*k5*x4)/k1 }