I wouldn't worry about it too much, as a 4 is still a "good enough" score.
It's possible to get a very boring (to you) topic; I've had that happen before, too, and it is harder to write an essay about it. (I once got one - not on the GMAT - that asked me to discuss whether I thought politics and societal trends influenced fashion or the reverse. Ugh.)
The key thing is to be able to brainstorm a couple of good examples. If you've got 2 good examples, then you can still write a decent essay.
If you've got OG12, it has a list of essay topics in the essay chapter. You can use those to practice brainstorming examples. Choose a random one every day and spend 3 min brainstorming. If you choose one, read it, and think, "Yes! I love this topic!" then skip it and pick a more boring one. :)
Also, if you write well (and you do, or you wouldn't have gotten a 6 on the other one!), then you can get away with just two examples, not three.
This article might be of interest to you:
http://www.beatthegmat.com/mba/2009/12/ ... -no-thanks