I'm just curious what your first is? I love all of them, but, come to think of it, for me 3 was second worst too... but I'm going to get flamed by some one, 1 was my least favorite (2 on the other hand was my 2nd favorite). *DUCKS*. I guess I'm surprised cause I find most people who didn't like three (not just it's not my favorite fallout like me) usually have it as last (if 4 isn't I guess).
I don't know where 4 falls in cause I find it a worse RPG/Fallout than 3 but as a game overall I do think I enjoy it more than 3. So I really don't know where to place it in my list of least to most favorite Fallouts. It's definitely behind New Vegas and 2 though (and above 1, sorry old school Fallout fans. I liked one ok but never really could get into it. And I tried. Hell, it was the first Fallout I tried to play and I have three different versions from three times in my life trying to play the game but just never got far).
But, on topic, I hear that this DLC does a lot of what I think 4 failed in so bad as an RPG/Fallout. I hear there is a lot of charisma checks that actually do affect the story and even skill checks. There's other issues I have with 4 but those are the two main issues that if they fixed, I'd be happy with it as an RPG/Fallout game (wouldn't be perfect but I would easily put it above 3 then. Hell, if they did it well enough, but I'm skeptical Bethesda has it in them, that could put it above even New Vegas. But I think they'd have to have better writers for that and also more free range to play whoever you want and not Bethesda's character. 4 is particularly bad about that and I blame the voice actor honestly. Without him/her I could still kinda fudge the limited dialogue options in my head to fit who I'm playing. well mostly). So if this DLC does what I hear it does, at least we have one DLC that makes 4 the game it should have been. I mean there is other stuff that could be better but for me they are small gripes I could overlook if the dialogue was better and ability to do stuff other than combat was there (and the ability to have something not combat related affect the quest storyline).
So I'm excited though I just started a survival character who isn't even close to being ready to do any DLC (and I hear it's best if you have met and gotten in with all the factions before you start Far Harbor). And I don't really want to be trying to play two characters at once (I haven't finished the story on my original character).