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