A couple of suggestions:
You can search BMI* which gets around the 4-letter restriction. Of course, it will return any word which starts with BMI, which isn't as much of a problem as if you were searching for ATL.

When you open the search window, select Advanced Search. Then scroll down to the IB Beta Search. The "Beta" search function does not limit the number of letters in a search.

There is more info. in the "search" sticky at the top of this forum: http://www.flyertalk.com/forum/showt...00#post9115800

and, in this thread.