Here's another Code 4 finding. When you go in (or should I say, when it lets you go in) without logging in on the first map of the day, I've found it will sysdump you when the game does it's sweep. Apparently, the game sweeps the server/connections at some interval (havent counted it out yet). If you are in there on an old log in, not of the current date, it will dump you to your desktop.
Tuesday, I went right in without logging in thru Nova, and was in a map for about 15 minutes, then got dumped to my desktop. Thinking it was lag or some stinking Vista background program restarting, I gave it little thought and went to eat dinner. When I came back I got a code 4, logged in and all was good.. Yesterday, I failed to log in (again) and went right in, only to get another dump 3/4 of the way thru a DM, and when I tried to get back in to finish the map, it went to Code 4 on me again. That's when I remembered that I had not logged in for the first map of the day and I realized that it had found me (again) during a sweep, and dumped me for not loggin in. Since Code 4's are only for log in failures, the two occurances now made sense.
So, the moral of the story is.... If you get a desk top dump during a map, it may be caused by not logging in for that day, especially if it code 4's ya when you try to get back in.