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