[ Eq(Derivative(x1, t), (1*k28*(1 + k17*k25/(k25 + k18))*k2*k4/(k4 + x4) + (-1)*k28*k1*x1)/k28),
  Eq(Derivative(x2, t), (1*k28*(k20 + (1 - k20)*k18/(k25 + k18))*k2*k6/(k6 + x3) + (-1)*k28*k1*x2)/k28),
  Eq(Derivative(x3, t), (1*k28*k9*(1 + k21*k25/(k25 + k22)) + 1*k28*k8*x1*(k10 + (1 - k10)*k11/(x4 + k11))*(1 + k23*k25/(k25 + k24)) + (-1)*k28*k7*x3)/k28),
  Eq(Derivative(x4, t), (1*k28*k14*(k15 + (1 - k15)*k16/(x3 + k16)) + 1*k28*k13*x2 + (-1)*k28*k12*x4)/k28) ]