You can solve it yourself from scratch in a few minutes by running my Fhourstones benchmark [1]. Other interesting connect-4 tidbits may be found on my connect-4 page [2].
Yes, I may do a minimax version with a difficulty setting. That would be fun to do.
If you tell the optimal algorithm to only think 3 moves ahead, it's too easy, and if you let it think 4 ahead it becomes very difficult (but not quite impossible).