If I would compare NV to F4 in terms of linearity:
NV - more of visible walls, following the main road to the main quests, 3 main factions - fully involved in the main story, some minor hostile/non hostile factions more or less connected to the main story.
F4 - no visible walls, 4 main factions - fully involved in the main story, some minor non hostile factions minding their own business.
But in NV why players said it is non linear?
1. you had to opposite factions you could join
2. you didn't have radiant quests supposingly supporting their cause, while being only annoying, pointless and game breaking
instead in every city on your linear way to the main city, you could decide with your actions there if you will be on 1 side or the other, and you could decide to support minor hostile/non hostile factions.
So you had importing decisions on your way to the main quests, where it was all resetted and you started again new round with new decisions.
In F4 - it looks the same at first, but
1. you have 4 factions, where you do all the same crap for them over and over on your way to the main quest, so after some time, you are not sure at all what you do and why - I wouldn't call that a good distraction from the main quest, "non linear quests".
2. if the main factions are not the opposite, there would have to be some really good story behind with a lot of background, more like a detective movie, and I'm not sure if it is possible in a game like this - or it will be just grey in the end = not interesting. Well it could still be good if the side quests would be different and would lead to different places, would be always meaningful and not annoying.
3. you have a lot of hostile factions, that should have some impact on the world, but you can only fight with them in this game.
The rest is OK for me - all the world builded around.
I don't know why in TES games there could be thieves and murderers and in Fallout 4, where you would expect that, is nothing.