I was thinking about this earlier on what the last 4 DLC could be. The rumors so far suggest an alternate look for Ciri and possibly a New Game+ so what could the other 2 be?
Well with those rumors included this means we'd get 3 Alternate Look DLCs, 3 Quest DLCs, 3 Armor DLCs, and since 3 is the magic number here I would expect that to be all of those individual kinds. Additionally we have a new Witcher Gear set, new Crossbows, new Beard styles, and a Gwent card replacer. With the New Game + we're up to 14 and so my guesses for possibilities on the other 2:
-New Sword Sets: I kinda doubt this just because we already have such huge variety in game but since we got a Crossbow DLC I won't rule this out, though admittedly they needed a lot more variety.
-Storage Option: A place for you to store additional items, maybe at inns or the Witcher keep.
-New Brothel: At the Nilfgaardian camp we have The House of Respite, a "gentleman's club" for officers, while you can get in here now there's no real use for the area, but with a DLC you could add a few new prostitutes, some merchants, Gwent players, maybe even a quest or two.
-New sex scenes: Since we're on the subject, there's always the possibility that CDPR held back their most sexy stuff for a opt-in/opt-out DLC, this could be brand new scenes in addition to what's already in game or just more steamy versions of the ones we already have.
-Big Head Mode: Because we all have been begging for it... this is a joke actually, I am out of ideas.