In Skyrim and Fallout 3 and NV there were a couple of main characters that had specific voice actors/actresses but the remainder of the characters were typically divided up among 4-5 male and 4-5 female voice actors. Meaning there was a lot of repetition in the character's voices due to recycling the actors. That doesn't seem to be the case with F4. The IMDB page lists over 100 voice actors each with no more than 4 characters. Most only voice 2-3.
What this means is that when creating new NPCs for F4 there should be 15-20x more voices to choose from already stocked within the creation kit when assigning your NPC's a voice and dialogue. Hopefully allowing a great deal of diversity for NPC related mods.