[ Eq(Derivative(x1, t), (1*k20*k1 + 1*k20*k19 + (-1)*k20*k2*x1**k3/(k4**k3 + x1**k3) + 1*k21*k5*x2**k6*x1**k11/((k7**k6 + x2**k6)*(k8**k11 + x1**k11)) + 1*k21*k9*x2 + (-1)*k20*k10*x1)/k20),
  Eq(Derivative(x2, t), (1*k20*k2*x1**k3/(k4**k3 + x1**k3) + (-1)*k21*k5*x2**k6*x1**k11/((k7**k6 + x2**k6)*(k8**k11 + x1**k11)) + (-1)*k21*k9*x2)/k21),
  Eq(Derivative(x3, t), 0/k20),
  Eq(Derivative(x4, t), 1*k20*k14/x3*(k13*x1**k16/(k15**k16 + x1**k16)/k14*(1 - x4)/(k17 + 1 - x4) - x4/(k18 + x4))/k20) ]