It didn't work for me: when I tried to open the map as a map, I got a "map wrong size" message and the map was totally blank. But if I opened the map as a scenario, it worked...weird.
Same. What I do, when converting stuff from III to IV is, to load the scenario, dump the map, .OOB. Alas, when designing or modifying/adjusting old scenarios, I make sure I use III, e.g. OOBs for sure with the help of Andy's editor and Notepad++.