For there is an inverse relationship between vessels and lights. The first vessels grow with each Partzuf, starting at the beginning. But the lights are the opposite, for the lower lights are enclothed in the Partzuf at the beginning. Hence, if the vessels contain only the upper [levels], namely ḤaBaD, then only the lower-most lights, those of Nefesh, descend there to be enclothed. That explains the first two thousand years, the years of chaos.
The second two thousand years are linked to the vessels of ḤaGaT, and the light of Ruaḥ descends and is enclothed in the world, which is the secret of Torah. That is why the middle two thousand years are called “Torah.” The last two thousand years are the vessels of NaHYM, when the light of Neshama is enclothed in the world. For that is the greatest light, and that is why they are the days of the Messiah.