Going to fourth density? Or fifth?

[anagogy]

What does 'congruent to a great extent with yellow ray' mean?

In my opinion, this means that the 4th density environment resembles to a great extent the 3rd density environment. It's like a higher vibrational version of the same environment, well, that is to say, until it is fully formed. At which point, it will not be molded by yellow ray, anymore, it will be the other way around.

Also, I notice how the 4D sphere coexists; the 3d sphere will cease to be inhabited, but it doesn't go away... Do they share a common space/time tho?

In my opinion, no. It is a completely different plane of existence. A separate set of space/time geometry.