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