[ Eq(Derivative(x1, t), (1*k10*k1*x1*(1 - (x1 + x3)/k2) + (-1)*k10*k3*x1*x3)/k10),
  Eq(Derivative(x2, t), (1*k10*k3*x1*x3 + (-1)*k10*k3*exp((-k9)*k4)*x1*x3)/k10),
  Eq(Derivative(x3, t), ((-1)*k10*k3*x1*x3 + 1*k10*k3*exp((-k9)*k4)*x1*x3 + (-1)*k10*k5*x3 + (-1)*k10*k6*x3*x4)/k10),
  Eq(Derivative(x4, t), (1*k10*k7*x3*x4 + (-1)*k10*k8*x4)/k10) ]