Could be both, enemies scaling with you as well as zones of relative tougher enemies. Kind of immersion breaking either way. What, were all those mole rats keeping out the death claws?
If they do have zones of tougher monsters, there should be some at least superficially logical reason for this.
Whatever happens in regards to level (and/or zones) scaling in vanilla Fallout 4... rest assured (saying this to everyone) that the modding community will fix it. I suppose it's possible that vanilla Fallout 4 will already be balanced, and that the devs might have found a good 'middle ground' for scaling, especially past the game's ending in order to just continue on playing essentially forever. But I have the confident gut feeling that it just isn't possible due to Bethesda's track record. And what I mean by that is that Bethesda basically relies on the modding community to either 1) fix the game for them (and think about it, they save money in the process) and/or 2) balance the game for them.
The unfortunate part is of course time itself. It takes time for modders to adjust (and readjust), to change, to "balance" and even sometimes fully remake any specific game mechanics (such as levels scaling). So, assuming here just a moment, for the sake of discussion, that Fallout 4's vanilla status in regards to levels scaling ends up being "screwed" and entirely imbalanced, or simply 'proportionally scaled with player all the time', which would be the worst case scenario (and especially so after the game's ending) then 1) yes, it will be fixed by mods... but, 2) it will take quite some time, lest you just jump on the community-made equivalents to "early access" of unfinished "test" releases with odd version numbers like 0.1a.
I have to admit that out of all the issues that Fallout 4 will have (not "if", but will) if there's one I fear the most is indeed a constantly-following levels scaling going along with the character's as you "progress". That one alone, for me, would be basically a show-stopper, would go as far as saying game-breaking. I remember having to (I did say HAVING to) mod that sort of proportional scaling out of Oblivion in order to not only enjoy the game but merely to just continue playing it.