system
Given the following post, respond as if you're commenting on Reddit:
human
How can we have 4D objects?

It was my understanding that 2D used an axis on x and y...
then 3D used an axis including a third dimension, Z.

So how do we have 4D? Can we have 4D?
gpt
It depends on what you call a dimension and what you call an object. In your example you have X,Y, and Z and you want to add another dimension (let's call it Aa and work from there), while that seems to be impossible to draw we could visualise it.

Imagine you are in a room, this room has three dimensions (X,Y,Z) and a bunch of other variables (heat, pressure, filled with a solid, liquid, or gas, you, light density...), for every point in the room you could determine these variables and draw them (using a computer) in colour. You now have a four dimensional drawing of an object.

Some people call the progression of time a dimension, here if your object changes over time it is a 4D object.

Now for some harder science, quantum physics tells us everything is a wave. And that every wave is orthogonal to every other wave (minor simplifications here, but the example still stands) and that there are infinitely many waves. So you have a wave oscillating in the X axis, one in the Y, one in the Z, one in the Aa, and so on until eternity. 
Of course we can't see all these dimensions, but the maths say they exist. And really in science that is all you need.