I think you're giving up too soon, Steve. It has nothing to do with being smart enough. It's the amount of time you put into it. You might find that the people who have a very good grasp of +4D geometry have been studying it for more than 5~10 years.
The type/level of math one has access to can also be a limiting factor, which relates to the sophistication of the visuals one can make (the diff between an approximation of your 'best guess', and a direct mathematical output of a function, for some hypershape) Another thing that can happen is not enough collaboration with others who study the same thing. Often times, you'll run into a barrier in reasoning, when it just doesn't make any sense. This is when a lot of people give up, when they should be posting more questions than they have answers for.
Instead of "Just give up everyone, it's impossible!" , maybe try a "Alright, this is as far as I can go with my reasoning. Anyone have an idea, or a visual that might help?" . One leads to greater knowledge, the other does not.
We are embedded in a 3D physical reality, this is true. Any attempt to grasp a 4D object is met with great resistance for some time. But, you can still use a little 3D intuition and 2D -> 3D analogy to help the process. The truth is, a 4D physical object is about as alien as it gets. They're truly alien to our 3D understanding. We will never come across anything like a tesseract passing through a 3-plane. But, we can create a tesseract in 3D slices (or 3D projection) and play around with it, on a computer.
In addition to that, the very best way to study such a thing is to reduce these different n-D shapes into a math expression, and work out the finer details from it. On this forum, there are several forms of math that do this. This method is far superior to visual-only, or best guesses. The exploration you've taken is a great intro that will help with further study. You've got the basic concept down, regarding how flat a 3D object is in a 4D space. That's further along than most other people.
So, don't stop here with your interest. I also recommend giving yourself some mental breaks from such intense studying/visualizing. It is exhausting, trying to imagine something clearly for the first time. Your mind can't always be 'on' the whole time.
in search of combinatoric objects of finite extent