I don't have anything to be let down on. Either it MW or CW BoS. I would prefer MW, as it gives more lore options and will be a way to fill in a huge chunk of timeline from FOT to FO4. If it isn't, I'm not going to be let down. I would be slightly disappointed at a missed opportunity.
IF the game is 2277, it is clearly MWBoS. If they set game a couple to few years later, I wouldn't be surprised if it is MW AND CW. Nor would I be surprised if it was CW alone. I wouldn't even be surprised if it was WC BoS.
Part of the reason for having an ongoing franchise and one with deep lore, is to touch base on previous games. We still don't know what is canon from FOT, and this is an opportunity to do so. If they just continue the story from 3, without any mention of previous games, it is another FO title with no data on previous games, just like 3 had almost no data, which is something I feel the series needed to change, and the 4th game had to give lore info on Tactics(and 3) in order for it to really feel like a FO game from back in the day.
Now, I have a bad feeling that while we may get info on Tactics(If it MWBoS), we may not get any lore from 3(because of the date). I would prefer lore on both, and I would prefer both MW and CW in game. But I have a feeling we will only get lore for one game, with maybe some kind of mentions about the other. For example, if it takes place in Oct 2277, there may be some new about earlier events in 3, maybe even a first edition wasteland survival guide fresh off the press, and some mention about a water purifier up and running, but that is stuff we already know so meh.
So, I don't really care, but I am leaning for what would be more interesting and give the most lore, which would actually be both factions. It isn't like I have a favorite BoS faction, I enjoy the lore as a whole and want as much of it as possible in 4, seeing how 3 was very light on previous lore. IE Enclave lost(no duh) and hey look there is Harold.