An add-on can cause a high spike in memory. As I mentioned, there are a host of reasons why an OOM can occur and can occur even in quick fashion. Some things I've discovered have not even been mentioned on FS forums.
Bottom line, unless you do actually have some serious issue with an add-on, the 4gb is enough to handle virtually anything you can do or use in FS9 and not have it crash. FSX, that's another story! haha