[ Eq(Derivative(x1, t), ((-1)*k1*x1*k18/(x4 + k18) + 1*(1 - k9 - k15*k16/(x4 + k16))*k6*x1*(1 - (x1 + x2 + x3 + x4)/k5))/k21),
  Eq(Derivative(x2, t), (1*k9*k6*x1*(1 - (x1 + x2 + x3 + x4)/k5) + (-1)*k2*x2*k19/(x4 + k19) + 1*(1 - k13)*k10*x2*(1 - (x1 + x2 + x3 + x4)/k5))/k21),
  Eq(Derivative(x3, t), (1*k13*k10*x2*(1 - (x1 + x2 + x3 + x4)/k5) + (-1)*k3*x3*k20/(x4 + k20))/k21),
  Eq(Derivative(x4, t), (1*k15*k16/(x4 + k16)*k6*x1*(1 - (x1 + x2 + x3 + x4)/k5) + (-1)*k17*x4)/k21) ]