[ Eq(Derivative(x1, t), (1*k25*k8/k9*x1*(1 - x1 - x2) + (-1)*k25*k11*k24/k9*x4*x1 + (-1)*k25*k10*k14/k9*x1*x3)/k25),
  Eq(Derivative(x2, t), (1*k25*k10*k14/k9*x1*x3 + (-1)*k25*k26*x2 + (-1)*k25*k12*k24/k9*x4*x2)/k25),
  Eq(Derivative(x3, t), ((-1)*k25*k10*k14/k9*x1*x3 + 1*k25*k13*k14/k14*x2 + (-1)*k25*k15/k9*x3)/k25),
  Eq(Derivative(x4, t), (1*k25*k16/(k9*k24)*exp((-k19/k9)*t) + (-1)*k25*k17/k9*x4)/k25) ]