{ diff(x1, t) = (1*k15*k10*x3*(k11 - x1)/(k12 + k11 - x1) + (-1)*k15*k13*x1/(k14 + x1))/k15,
  diff(x2, t) = (1*k15*(k1 + k2*k16) + (-1)*k15*(k3 + k4*x1)*x2)/k15,
  diff(x3, t) = (1*k15*k5*x2*(k6 - x3)/(k7 + k6 - x3) + (-1)*k15*k8*x3/(k9 + x3))/k15,
  diff(x4, t) = 0 }