i guess 480x480 is hostable because it's only the size of the map itself that matters, not the bounds.
As long as it's max 4 MB (8 MB now actually)...it's hostable.
You can try this: make a 480*480 map, import some mp3`s to make it larger then 4MB (8 if you have 1.24 patch) and see if it works. It should.