Dictionary?
I like this concept and it seems like it would be a great game but one major issue is holding it back. The user has to guess exactly what word the game has in mind, granted their four letter word combination (on the first level) but there are lots of words that share letters especially with 4+ letters.
For example, the word that killed me was "vine" - I didn't get to vine quickly enough because I spelled vein, then spelled vein again thinking I did it wrong.
NEVI, VEIN, VINE are all 4 letter words in the English dictionary. There needs to be a way to tie this game into a better dictionary (maybe some kind of open source scrabble thing) so that it recognizes all combinations of 4 letter words and not just the one the game is "secretly thinking of"