[ Eq(Derivative(x1, t), (1*k13*k11 + (-1)*k13*k4*x4*x1 + (-1)*k13*(x1*k3*x2 + x1))/k13),
  Eq(Derivative(x2, t), (1*k13*(x2*k6*x1 + k8*x3) + (-1)*k13*(x2*k2/k1 + x2*k7*x3))/k13),
  Eq(Derivative(x3, t), (1*k13*k4*x4*x1 + (-1)*k13*k5*x2*x3)/k13),
  Eq(Derivative(x4, t), (1*k13*x4*k10*(1 - k9*x4) + (-1)*k13*k4*x4*x1)/k13) ]