The 4th unanounced game mode seems like it's going to being some type of co-op horde like mode. Check the link.
The video is old (like a couple of months ago), but I stumbled across looking for a Zeus trailer on Youtube because I was sad that @Just-Tank confrimed that there would not be any type of "clash system" or "saber lock" within the game.
BUT THIS CHEERED ME UP!
Just looked at a quick documentary by the Dev Team and this seems to be the intro for the Dessert of Lost Souls!
I'M STILL STOKED THOUGH!
Hate to burst your bubble man but that sequence was made specifically for when they revealed the Multiplayer to the press for the first time.
They wanted to misdirect the press so when they showed the full 4 vs 4 player match it really surprised those in attendance.
But there is some good news. I agree the 4th mode could be some sort of co-op versus enemy monster type gameplay due to this very recent video:
Not only does it show two players versus a cyclops in a test arena (which could be from the press reveal) but around 3:47 there are Saytrs attacking a player.
Unless some of the levels allow players to unleash enemies (which would be awesome) it looks like there could be a mode where player fight against the enemy creatures.
I'm not 100% sure that will be the 4th game mode but I think there is definitely a possibility!
It's most likely a co-op mode, you have to look at other games to figure out what could work, any type of TDM won't be in since people already treat TFoG and CTF as much, more so CTF, bomb assault I don't think could work cause it would never end and just be a massacre fest with both teams trying to get bomb. I'm thinking they can do it in survival mode, wave/round mode or "bloody palace" mode, most players who don't like the MP aspect can at least get into a co-op aspect of it