It's worth re-emphasizing that the definition of a 4D stop-gap between 3D and 5D is an interpretation from The 9D Pleiadians and as explained by the link in your question, those interpretations of where density/dimensional borders exist are pretty subjective...like people trying to decide where the color Turquoise ends and the color Blue begins.

But bearing in mind that subjective interpretation, the 9D Pleiadians are saying that the ultimate target for the current transition is 5D anyway so we're not going to hang around in 4D no matter what.

I think the amount of time we are spending there is just the minimum that is required to make it a comfortable switch.

They are saying that the transition straight from 3D to 5D would be too harsh. 5D has, for example, fundamental changes like parallel realities all occurring (being perceptible) at once, rather than serially (in 3D)...a fairly mind-blowing shift if you think about it :)

So they say that 4D - which we shifted into within the last couple of years, I think - allows the best of both worlds as a transition level.

4D is malleable enough to allow the continued projection of 3D "rules" upon it, for those who still want that experience, while also simultaneously allowing the projection of 5D "rules" for those who want a more 5D-like experience now e.g. with time being more "fluid".

As our experience of linear time goes on, more and more humans will start to experience 4D as 5D anyway and eventually I think there will then be a natural easing into the 5D experience.

The 9D Pleiadians have emphasized that the timescale they are giving of about 70 years is really just a broad guess. There is no linear time in 5D anyway and they have no idea themselves about the details of how the shift will occur (since there is no fate) so this is just a figure given to satisfy our 3D mental perspectives that demand facts and figures.

The timescale could change as we (as a collective) change.