They do not necessarily meet humans and try to interact with them. This question is more about the technical possibility/impossibility of 4D aliens being able to "analyze" the inner structure of our universe, mapping the galaxies, stars, planets, understanding what makes it expand, and search for signs of life, if possible.
The make-up of the 4D universe -- Based on this chart, I chose the green area in the graph to describe a realistic 4D universe: This universe may have two or more time dimensions, but for the moment let's just say that the graph describes a universe with stable matter and energy somewhere on the "green area". edit: a location with 4 spatial dimensions, yet may contain more than one temporal dimension if deemed necessary for a stable iniverse
The setup -- Our universe (named "the fabric"), is part of a higher-dimension universe in which the 4D aliens live. It happens, too, to be in the vicinity of the alien's world, and therefore they can easily reach the area and attempt to study the fabric. The alien's size scale is roughly the same as that of humans, yet in four dimensions: Their "end-to-end" length in any direction is no more than just a few feet.
The challenges -- At first, you may think of "Flatland", but the similarity related to visitor from a higher dimension ends here. While "Flatland" may appear philosophical or as a thought-experiment in nature, my question relates to the actual challenges of the physical world.
Can they see the inside? Matter and energy in our fabric cannot leave the fabric naturally. Not even light. The result is that the aliens may or may not see the fabric itself, yet they cannot see what's inside it. Based on that, what our fabric may look like to them?
I doubt they can pierce the fabric with either matter or radiation passing through it. By radiation they "X-ray" a portion of the fabric and see what's inside it. Their X-rays are a 4D analog of our X-rays. My reason to doubt it is here. This article describes colliding universes, with ours being one of them. While researchers claim tell-tale signs of collision, the universes were never observed to intersect or merge with each other.
Their size scale, similar to our, means they cannot "X-ray" huge portions of the fabric (if that was possible) and see whole planets or even galaxies. They must find a way to pierce through the fabric and send a probe with a telescope, but this raises the next challenge below.
The fabric is one spatial dimension lower than the alien's world, so its 4th dimension thickness is zero. There is no way the aliens can build such a probe. It is for the same reason that no one can build a 2D probe which can enter "Flatland", but we can only go through it. A regular (4D) probe the aliens have built can "fly" its camera through the surface of the fabric, but that attempt is of no use. The probe can never align to remain at zero distance in the 4th dimension from the surface of the fabric and I don't think something of zero thickness would show-up on the screen.
Based on all that, how the fabric would look like to our 4D aliens? What are its physical properties? Can they find a way to see what's inside it?