[ Eq(Derivative(x1, t), (1*(1 - x4)*(k1*(x2*1/2)**4/(x2*1/2 + k7)**4 + k10)*x3 + (-1)*k2*(x1*1/100)**2/((x1*1/100)**2 + k8**2))/k12),
  Eq(Derivative(x2, t), (1*k3*(1 - k9/(x1*1/100 + k9)*1/(1 + k11)) + (-1)*k4*x2*1/2)/k12),
  Eq(Derivative(x3, t), ((-1)*(1 - x4)*(k1*(x2*1/2)**4/(x2*1/2 + k7)**4 + k10)*x3 + 1*k2*(x1*1/100)**2/((x1*1/100)**2 + k8**2))/k13),
  Eq(Derivative(x4, t), (1*k5*(x1*1/100)**4*(1 - x4) + (-1)*k6)/k12) ]