If we're using Bethesda as our measuring stick, then we really need to start with Morrowind (or even Daggerfall) as our start point. When we do that we see an unpleasant path being taken.
We started out with maximum sandbox. Morrowind was a game of absolute minimum direction. You could play that game for weeks on end without ever discovering that there even was a main quest buried in there anywhere, and you could easily ignore that quest for years - or stumble into on accident and leave again without ever noticing.
The Oblivion came out and while the sandbox was still there, a certain amount of guidance had been added. Now we had waypoints, fast travel, and quest markers. While we could certainly distract ourselves with side quests galore & ignore the main quest for quite some time, it was still hovering in our journals and nagging at us until we got back on track.
Skyrim follows this same pattern, too - until you get the DLC. Once you install Dragonborn you've got that prick jumping your dragon kills and effectively halting your Shout progress until you head over to Solsteim and finish him off. If you install Dawnguard then you can no longer walk into a town at night without a bunch of random vampires showing up and killing off all the NPC shop owners and generally borking up your entire game thus; again until you go do the dang quest and put an end to it. In other words, they stop using the proverbial carrot and instead go for the stick. You are actively punished for not leaping right into the DLC main quests.
And, as other have already pointed out, Fallout 3 offered more player freedom than Fallout 4 did. The this pattern is consistent so far in that franchise as well.
What does all this mean? It seems rather obvious - the sandbox element is being slowly replaced with themepark.
You may notice that this is exactly what has happened with every MMO over the years. Where once there were sandboxes, now there is nothing but themeparks. Even EvE, the last "true" sandbox MMO out there, has been slowly but surely adding more and more "quest" (read: themepark) based content. More and more "leet loot" that can only be acquired through scripted missions and "story events."
It is my belief that this "themepark > sandbox" mentality has been bleeding over to the single-player RPG business as well. In fact it seems to have creeped it's way into every sandbox game over the years. Even city building games have "missions" attached to them these days, and "Sims 2 Castaway" was someone literally turning a sandbox game into a themepark game. I could list more examples but I'm sure you already see the pattern, now.
So with that in mind, I am sad to say, I don't have high hopes for Fallout 4 having the level of sandbox game qualities that we want. If patterns & trends are any indication, it will probably slot the player down it's main quest from the start and not let you off until it feels like letting you go play outside a bit to level up a little, then drag you back inside for some quality time with the Big Story - the ending of which will be completely scripted and the consequences of your choices even further stripped away than ever.