How do I replace the frameset from JavaScript, say to go from 4 to 3 frames?

You can get access to the document of the top window using top.document, and write a new frameset HTML using top.document.writeln.

Lets say initially there were 4 frames in the frameset. You get URL corresponding to each frame using

URL0 = top.frames[0].document.URL;
URL1 = top.frames[1].document.URL;
URL2 = top.frames[2].document.URL;
URL3 = top.frames[3].document.URL;