(1) A 2D surface does not need to be raised into 3 dimensions before you find an edge.
(2) I don't see why you see it necessary to imagine the universe as a curved volume. However, if it were, I don't see that as being a problem as - like most people - I am capable of imagining such concepts.
(3) A balloon's surface does have an edge - the point where expansion begins, the squiggly bit you tie up. Either locate a related element in the universe model you are using or chose a more suitable analogy.
(4) You appear to be considering the balloon's surface as if it were an edgeless plane in 2 dimensions only. Fair enough if you wish to discard one dimension, but that is not an accurate model of the universe.
(5) To claim the universe requires a 4D brain to imagine it is a bit vague. Define '4D brain'.
And explain why my puny brain can visualise and manipulate a model of a hypercube if what you say is true.