Becoming Closer
Four Angels, Four Corners
nFour Trumpets?
nFour Corners
•common speech
•geography of the Seraphim
There is some argument -- it varies by commentator, not by theory -- that these four angels are in fact the first four angels of the Seven Trumpets.  This is due to the fact that those seven are grouped four and three.  It also appears that when you compare the judgments there is some similarity.
The real key is that these angels are at the four corners of the world -- which to John would have meant the Roman Empire.  Remember, this is a book written to Christians.  We do not need to seek explanations in North America for this..
The argument for geography, rather than symbolism, is well accepted by all points of view.