Man, what a wild ride! So the biggest thing we can do is narrow things down.
So Fighter 4 will give us a hint as to who Fighter 5 is. If this is the case then lets break it down. We all know its SNK, but lets test something.
If it is Ryu Hayabusa or Doom Slayer that is revealed, then the other is likely the next. This seems the less likely now, but could still blindside us.
If it is Terry Bogard, then Hayabusa is 5. Seems likely...But...
If it is Female fighter with flying weapon (Mai, Nakoruru or Athena), then fighter 5 is someone who is already in the game (Spirits do not deconfirm a potiental fighter)
I missed something...I will fix this. FIXED!! Sabi mentioned 4chan post, so Milkman "leak" is dead. Unless that was on 4chan at one point.