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