I really don't know much about this (only what I've read online). Your lens is original to your camera; so long as the film plane does not change when you convert to 4x5, there should be no problem (though you would want to test to make sure everything is correct).
Using a different lens will require you focus with ground-glass (and perhaps put a focusing scale on the bed), and/or recalibrate the rangefinder (the Byron site discusses this, especially under a link for grinding a cam).
There are various "how-to" pages on the web from people who have done this conversion themselves. There used to be some how-to pages for the Byron (I've not checked in a while, but I imagine they are still available).
Another conversion is the Razzle, but I don't know if it exists anymore.
There is also the Alpenhause:
http://www.alpenhause.com/index.htm
You've probably seen a classified here on APUG for one of those: (there was a url link here which no longer exists)
However, I get the impression you may want to do this yourself, as opposed to buying a pre-converted camera or sending yours off for conversion.