[ Diff(x[1], t) = (1*k[9]*k[7] + (-1)*k[9]*k[8]*x[1] + (-1)*k[9]*k[1]*x[3]*x[1])/k[9],
  Diff(x[2], t) = (1*k[9]*k[1]*x[3]*x[1] + (-1)*k[9]*k[2]*x[2])/k[9],
  Diff(x[3], t) = (1*k[9]*(1 - k[5])*k[3]*k[2]*x[2] + (-1)*k[9]*k[4]*x[3])/k[9],
  Diff(x[4], t) = (1*k[9]*k[5]*k[3]*k[2]*x[2] + (-1)*k[9]*k[4]*x[4])/k[9] ]