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