Is there a need for death in fourth density?

[Infinite]

However, it seems to me that Ra often uses the Law of Confusion and that the messages need to be interpreted.

I don't believe that. Ra was always clear in his answers, there was no kind of interpretive game. Only in one or the other answer did they leave it to the questioner to reflect. Ra's communication guidelines were clear: speak everything at the limit of language and retain only that information that would be harmful to free will.

My interpretation is that death is only necessary up to third density. Why? Because in those lower densities there is the experience of separation. Without separation death would mean that it's still the same one reality so death becomes kind of unnecessary in higher densities.

I think we can define death as the infeasibility of a physical vehicle that inhabits a continuous space/time. As according to Ra there is some kind of polarized system s/t and t/s up to the sixth density, death is a possibility to some extent of the sixth density. However, I agree with you that death only carries some traumatic load up to the third density. Above our current density there is no veil of forgetting and time/space is as noticeable as space/time. Under these conditions, death should be just like changing clothes.

Not only that. I also believe that we can move into fourth density without having to physically die first. Again, it's because all reality is one. So fourth density is simply about removing the veil of separation and making matter and energy vibrate on a higher frequency with more density of information on earth and in our physical bodies.

Well, densities can be defined as vibrational spectra. It seems to me that a type of vehicle more filled with light is needed to live in the conditions of the fourth density. However, I wonder if the entities that reached the highest possible point in 3D could somehow "project" themselves to the fourth density (just as we projected onto the astral plane). My theory is that it's necessary to have a body made of the same true color of density in which one wishes to live.

So one guess I have is that 90,000 years means the approximate time for fourth density to reach fifth density, without the need for any physical death of the body.

Ra spoke of the duration of a fourth density cycle that is around millions of years and in the material it's quite clear that the only short density is the third.