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