The way it works in Fallout 3 and Skyrim is that each location has an "encounter zone", which sets the minimum and maximum levels enemies will scale within at that location. For instance in Skyrim, Dragon Priest ruins have a minimum level of 24, which means the game won't spawn enemies under that level no matter when the character reaches that location. Whereas a more beginner-oriented location, like Embershard Mine, would only scale between levels 6-10 or so. And most enemies have a set level; level scaling in Skyrim mostly decides which version of the enemy spawns (Restless Draugr are level 6, Draugr Wights are level 13, Draugr Scourges are 21, etc).
The way they've described scaling in Fallout 4 doesn't sound any different to how they handled it in Skyrim, but they've been getting much better at it with each game so I'm optimistic. The only problems with scaling in Fallout 3 was Deathclaws appearing in random leveled spawns, and a lot of what Broken Steel added. Skyrim's only problem, even after all the DLCs, was they had to scale dragons to keep the challenge consistent and that made the world a little less consistent.