{ diff(x1, t) = 0,
  diff(x2, t) = 0,
  diff(x3, t) = (2*k2*k6*x4 + (-1)*k3*x3^2 + (-1)*k4*x3*x4 + (-1)*k5*x3)/k1,
  diff(x4, t) = ((-1)*k2*k6*x4 + 1*k3*x3^2)/k1 }