Well, it's updated now. Exactly how, I don't know! I did do the copy the xml file to an SD card, and indeed it installed the map happily on that. Moving the files around would have been a right palaver though, so held that in reserve. I played around with my normal methods of upgrading, that have always previously been successful, no luck. Then I tried taking the XML file from the Zumo that had upgraded successfully (having 16GB internal storage) and putting it on the 4GB Zumo. Told it to reinstall the map, and it went ahead and apparently spent the usual 15 mins or so doing it. But in the end, no sign of the new map files anywhere, so goodness knows what it was doing!

Then I deleted the XML file, let the Zumo recreate a new one, all by itself. When it had done that, I went back to my normal process of changing the ID in the XML file to that of the 16GB Zumo, making the XML file read only, and trying again. This time, for whatever reason, it worked! No fuss, just installed the update, no dialogue about size or regions, just went ahead, split as previously between the device and the SD card. Unlocked the maps and changed the ID back to the correct one in the XML file, all done. What the difference was, I have no idea - something in the XML file obviously, but what, I don't know.