Thanks again for all the kind words.
Today I traded email with the USGS and asked if they have similar topo line data I can display when the user zooms out 1 or 2 levels. Alas, they do not. I pointed out how well received their topo line data is in forums like this one and recommended they provide topo line data for the next two broader zoom levels. Hopefully that will happen at some point.
Joseph, the Gmap4 guy