Unit four appears to me to have no significant details left unknown.
This was a weak hydrogen blast. The hydrogen had accumulated slowly after electricity was lost and air circulation stopped. The upper portion of the building was becoming quite warm which was encouraging the chemical reactions which probably initiated the explosion.
Two clips from post # 856 by fusefiz:
Unit #4 probably exploded with near the minimum of hydrogen possible for a deflageration and far below the limit for a detonation.
Hydrogen reacts with enough metals and such so as to provide a source for the few joules of energy needed to set it off.
Compared to what most people have in their minds as an "explosion" this was more like a big puff. (I think most people visualize something high velocity such as an IED when they think of an explosion.) Fuel/air explosions require a bit of an adjustment of outlook for some.
But I see no secrets left at unit four.