Kachiko seems to be right. I've ran into a few more questions like this since the first, and they all follow the same pattern. Which is fantasticly easy when you know the pattern, but insanely silly when it's the first time you've seen it.
Pretty much start with the last 2 letters of the first word, then work backwards on the second until you find a 4-letter word.
If you don't find one, then move one spot left on the first word, and do it again, repeat until a word clicks.
Seems like more of a "have you seen this before" problem than a real "discover the pattern" problem.