[ Diff(x[1], t) = 0,
  Diff(x[2], t) = (1*k[2]*k[3]*k[8] + (-1)*k[2]*x[4]*k[5]*x[2]/(k[4] + x[2]) + (-1)*k[2]*k[7]*x[2])/k[2],
  Diff(x[3], t) = (1*k[2]*x[4]*k[5]*x[2]/(k[4] + x[2]) + (-1)*k[2]*k[6]*x[3])/k[2],
  Diff(x[4], t) = 0/k[2] ]