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