Here's a start I created to help address this question:
http://projectavalon.net/forum/showthread.php?t=16911
My understanding is on the Astral, Astral realm is fourth period. It is represented as the water element. The fifth doesn't have an 'astral' they'd have to go down to the fourth to get to the astral.
Astral is not where truth is found, that's found in the spiritual, the realms of pure Being in the true 7th and above.
When ET's say they are from 12 D or whatnot, they are actually in pockets of the 4th or 5th generally speaking. They are either lying, or they are counting various pockets within the dimensions as dimensions themselves. They use a high number to sound very advanced. Beings in the true 7th plus are infinite spiritual beings, such as Buddhas, Christ Beings, Angels, Elohim (not ET Elohim, true Elohim) etc.
Each dimension adds layers of complexity to our experience. For example, we have 5 senses in this dimension (not including the pineal gland, our 3rd eye, our sixth sense), whereas in higher dimensions we will have many more senses, like I'm guessing on this one, but 50 senses in the sixth kind of thing. Life becomes deeper, more complex, more real, more beautiful.
The earth is NOT ascending to the 4th dimension. That is the astral realm, it is a fallen dimension and the 7 levels of hell are in the lower levels of the astral. We'll be ascending to a new 5th (a new heaven and a new earth right?), as there is a great in breath happening and everything is moving higher.
We will move back on the divine timeline where it is much more clear that all life is One and that "God" / the Source / the Father is accessible directly within us.
However, contrary to what some ET's might want us to believe, there is no free ride. Our karma doesn't just get erased or anything and there's no such thing as automatic salvation / ascension.
So those that think we just have to welcome the et's and then we get to ascend, or that we go to the right church and 'we will be saved', I personally believe this is deception.
Whatever we have created karmically it is our duty to balance. We must take control of our consciousness and help the rest of planet earth make it through this time in order to earn our right to ascend higher in the mind of God. This is the right handed path, the path of truth, Oneness, reality.
When people say you are multi-dimensional, they're not lying. You have a body in each of the higher realms. Part of the process of ascending is purifying / enlightening these higher bodies.
The best book in the world (I say with little hesitation, although of course this is my opnion), to help you purify yourself and prepare yourself for this process, prepare yoru higher dimensional bodies is this one by Mother Mary through a messenger here:
Master Keys to the Abundant Life:
The cover is terrible, the content is beautiful. It will clearly explain to you what's happening in your higher bodies, in the higher dimensions and how to fix em up. It's a worthy read...
Further if you want to understand the whole game and reality to the highest degree, this book is mind blowingly amazing, the best I think that has been released to humanity up to this time:
Very few people have read these books yet, but that will change... I'm posting now in case someone here is called and ready.
As for which reality I choose to live in, I choose to live in Truth.