As far as population density inside urban areas, I think the technology has been demonstrated in Hitman to successfully generate and maintain crowded areas.
Here, for instance, is a Hitman video of a crowded night club.
https://youtu.be/1Fi2uD1SSck
Now, getting something like that to translate into NPCs that can be interacted with on a satisfactory level is another question, and might not be something the engine being used for FO4 is capable of handling properly.
It would, however, be nice to have several times more the population densities, at least in areas where one would expect a plausible grouping of people to crowd. Further, it'd be quite pleasant if the NPCs had a little bit more complex "life routine" and sandbox environment interaction with a scaled level of random built in. Certain characters would necessarily need be fairly static in their routines, but, some greater play in NPC transience could make for some fun.
Bounty hunting, for instance, if it's in the game, would be challenging if some of the target NPCs are actually on the run, laying low, and randomly pin-balling from wasteland camp sight, abandoned building, settlement, cave, and other places. In some ways, a bounty target could then do themselves in if they run into a nest of rad scorpions while trying to hide, or, even seek protection with a group of raiders, or some other faction.
That would be really nice, but, I doubt the engine is up to that level of sophistication.
Still, a peep can dream.