Well, first of all, I tend to think it's kinda misleading to use these kinds of labels, and say that those reptilian beings are from the "4th
dimension", or the "5th dimension", or wherever they may be from... Well, it seems that what David Icke is saying is that some of those beings are
living in a parallel dimension just "above" ours, which he seems to call the 4th dimension. (in many metaphysical circles, our world is referred to
as the "3rd dimension", and the levels above us are called the 4th dimension, the 5th dimension, the 6th dimension, and so on and so forth the
"higher" you go...)
As for the nature of those dimensions and the structure of the Universe itself, well, according to several sources of channeled material, space is
composed of a large number of "levels" interpenetrating each other, and each one of those levels (sometimes called "parallel universes",
"parallel dimensions", etc) are apparently located in a specific frequency band...
As an example, there is the dense physical matter we can perceive all around us (objects, people, animals, etc), and then when you move higher in
frequency there are several kinds of invisible things that we can't normally perceive (radio waves, ghosts, spirits, etc), and then if you go at an
even higher frequency, you eventually reach a point where you actually cross right into the next octave, which would be a parallel universe of its own
that is "above" ours in the electromagnetic spectrum...
So, according to all this, it would seem that the key to understanding those various dimensions is the frequency factor, the frequency at which some
particular matter is vibrating. I haven't read much of Icke's stuff, but by saying that these reptilians are in the "4th dimension", he probably
means that they are beings composed of matter oscillating at a frequency range that we can't usually perceive, thus, in an "higher dimension"...
Whew, anyway, I hope all of this made sense.
For those who want to get deeper down the rabbit hole, I definitely recommend this site
... Enjoy the ride.