[ Eq(Derivative(x1, t), ((-1)*k3*(k5 + k6*k1) + 1*k2*k14*x2)/k2),
  Eq(Derivative(x2, t), (1*k3*(k5 + k6*k1) + (-1)*k4*k7*x2**2/(k8**2 + x2**2) + 1*k3*k9*x3**4*x4**2*x2**4/((k10**4 + x3**4)*(k11**2 + x4**2)*(k12**4 + x2**4)) + 1*k3*k13*x4 + (-1)*k2*k14*x2)/k3),
  Eq(Derivative(x3, t), (1*k3*k1*k15 + (-1)*k3*k16*x3**2*x2**k19/((k17**2 + x3**2)*(k18**k19 + x2**k19)) + (-1)*k3*k20*x3)/k3),
  Eq(Derivative(x4, t), (1*k4*k7*x2**2/(k8**2 + x2**2) + (-1)*k3*k9*x3**4*x4**2*x2**4/((k10**4 + x3**4)*(k11**2 + x4**2)*(k12**4 + x2**4)) + (-1)*k3*k13*x4)/k4) ]