Is 4D less or more dense than 3D?

[Infinite]

But I don't know enough about the lunar/solar "bodies" and whether or not they may map cleanly to the Fourth Way "no real body, hence aligned with the moon" vs. "something which has fully transcended the limits of physical life".

One of the sources that speak of this solar astral body is the Gnostic movement founded by Samael Aun Weor. Interestingly, he relied heavily on Gurdjieff's Fourth Way.

Maybe that's the same as simply being conscious and able enough to know what to do next or where else to go in taking a step away from the old?

Yes, it is another way of stating what I said.

But how does growth of consciousness relate to the development of more fine subtle bodies? The Gurdjieffian answer would be that the qualities of consciousness depends on the quality of the instrument it uses, thus better subtle bodies enable a qualitatively better consciousness (in a way only faintly or not at all reflected at the level of waking physical life).

I haven't delved into Gurdjieff's information yet, but it seems to make sense. Thanks.