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