[ Eq(Derivative(x1, t), (-k9)*x1),
  Eq(Derivative(x2, t), k4*x2*(1 - (x2 + x3 + x4)/k10) + k6*x4 - k5*x2 - k8*x1*k9*x2),
  Eq(Derivative(x3, t), k5 - k8*x1*k9*x3),
  Eq(Derivative(x4, t), k8*x1*k9*x3 - k6*x4 - k7*x4) ]