JoaoPais said:
A more realistic view of a 4th dimension is to consider that, if you could look from it, you would see behind the walls, the inside of a box, or even the human body. Everything we see is in a 3D referential, that's why we have to go around a wall to see what's behind. In a 4D world you wouldn’t need to, because the wall is only an obstacle the first 3 dimensions, so in a 4th dimension, "light" from behind the wall could reach your eyes.
Exactly that was my favorite mental revelation from reading
Flatland! (It's a seriously thin, tiny book btw; Dover Thrift edition like $1.50! Great stuff!)
In it, the author illustrates how for the 1-D "Line Segment" people, their world is like burrowing through an underground tunnel. All you can do is move forward or backward until you run into an obstacle. If one Line-Seg ran into another, he would see the outermost surface facing in that direction: his "face." [And if he met him coming from the opposite direction...] So if the Line-Seg man is 10 segments long, others can see either of his two faces, but were they to wish to find out what (e.g.) color those middle 8 segments are, they would have no choice but to dissect him! Like chop off the face, note the color of the newly exposed segment in front of you; chop off that segment, note the next color visible, and so on, until the unfortunate specimen can (at least) be
conceptually reconstructed, and analyzed inside and out.
You'll notice, however, that observers with the aid of additional dimensions have no such problem. We, or even 2-D Square-Men living on a flat sheet-of-paper-like world, can naturally and instantly see the "hidden" inside segments of the Line-Man. We don't have to chop them up, we can just take notes. It's the same way for us observing the aforementioned 2-D Square-Men. To one another, their outer-facing borders are their visible faces, their skin. They could walk around one another to see all the exposed sides, but once again, if they are wondering about the inner working of their various bodily systems, dissection will be in order. Of course as Men of 3-Dimensions we can just glance down at them from above and see exactly what's going on in there, as it functions, without disturbing a thing.
Of course the punchline is to try making use of those analogies to imagine how a 4-D being might view us. It's not just that they could see "behind walls" and "inside boxes," but that they could see every spec of your inner being splayed out as plain as day. To me that was the only analogy that ever made me feel anywhere near to doing what is so obviously "just inherently impossible"

: visualizing what it might be like to exist in a 4-Dimensional universe... And now that I'm thinking about this again, looking more closely at exactly how (e.g.) a Line-Seg man might be able to glimpse what the Square-Man sees by nature (and so on) might be interesting.
And, it should be noted that the "compact" extra dimensions referred to by the OP
are different from the type discussed above.