May I use my analytic skills on this one please, because this thread is gott be the most insane thing I have read all day.
In an interview Boon has stated he cannot safely say there will be only 4 DLC characters, judging by the immense fan demand of MMH, plus all of his unofficial polls for fan favorites and his refusal to compleely reveal the winner of said poll and consider the current tweet, he is stating there is at least one more DLC character coming after the 4th planned DLC character. Lest we also forget the odd MMH tease at the end of the Scorpion trailer, also his tweet states "charcter(characters)", it is safe to assess that there is good 80% chance that there will be second set of DLC characters and an even higher chance there is at least one more on the way as well. Also consider the statement of it being fan requested and that is all he can safely say, also brings in another assessment that there will be more than one considering the polls that had been put forth.
Finally as for the who our 4th is, thus far the rumour leaks have been dead on with Batgirl and Scorpion, it is safe to asses that Zod is the 4th, also considering the MoS movie is on its way and his earlier tweet of him agreeing to a tweet directed to him saying "We need some MoS DLC for Injustice".
Long story short 4th DLC will be Zod and there will be likely 4 more DLCs one of which is going to be MMH.
Confusion Cleared Have a nice day Ladies and Gentilmen