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