I think your right on the money Stanley. It is a very interesting question and one I plan on looking into further.
The Magician's Nephew obviously first introduces us to Narnia and how the monarchy was formed under one king. I feel this king is the "High King", that is the title given to Peter in the proceeding books.
I get the feeling that Lewis didn't have any hidden or secondary meaning when concerning the four thrones. I also get the feeling that Peter was the King and the others were just noble servants under him. When others were king, such as Tirian (again, notice the one king factor), I feel there must have been others who possessed similar titles that Edmund, Lucy, and Susan were granted. It just may not be apparent to us because Lewis had no need to go into those characters as they are not central to the plot.
Not sure if that made any sense, just brainstorming since I have never really thought on this aspect of the book. Interesting though... :)