Well from the same guy who leaked Doomsday and the remaining DLC comes a new leak. According to him after digging around the season pass data he found this-
It seems that the meanings of the four in order are - Martian Manhunter, Red Hood, Power Girl, and Sub-Zero. Is it true or not? You be the judge.
EDIT: Thanks to Chainsaw for bringing this to my attention.
Scroll down a little bit from the top.
The above image is taken from the provided link. Again, please note that this is just a rumor and nothing has been officially confirmed yet.
Article edited by GamerBlake90