{ diff(x1, t) = (1*k20*k1*x1*(1 - k2*x1) + (-1)*k20*k3*x1*x3/(k4 + x1) + (-1)*k20*k5*x1*x4)/k20,
  diff(x2, t) = (1*k20*k6*x1*x3/(k7 + x1) + (-1)*k20*k8*x2 + (-1)*k20*k9*x1*x2 + 1*k20*k17)/k20,
  diff(x3, t) = (1*k20*k10*x1*x2/(k11 + x1) + (-1)*k20*k12*x3 + 1*k20*k16)/k20,
  diff(x4, t) = (1*k20*k13*x4*(1 - k14*x4) + (-1)*k20*k15*x1*x4)/k20 }