Where did it really improve... Graphics, obviously, and the gunplay. And the weighty feel of the power armour. But to be honest, that's about it. Everything else, like the writing, the amount of items, the character creation (not the facial customisation, Fallout 4 did that one better at least, but the actual stats and RP possibilities), the depth of every single gameplay aspect... Fallout 4 was a huge step down. It's really sad, because it's obvious that they recognised some of the improvements NV brought in over 3, but either due to time constraints or just general cluelessness they managed to butcher everything.