Dunno why people are making a big deal about the engine, still see various forums of people complaining its old. But, because Fallout 4 was made in 64bit and not 32bit (pretty sure this is why)...but Fallout 4 is so much more stable and way better performance than I ever got in Skyrim. In Skyrim, it would stutter a lot as it loaded in new areas/assets even on a 100% vanilla game. Modded Skyrim always performed way worse as well, especially with ENB attached. However, even my pretty heavily modded Fallout 4 with an intensive ENB performs WAY better than Skyrim did with mods+ENB and Fallout 4 doesn't even have stability mods out. Even using an ENB in Fallout 4, I still get great FPS, no stutters or anything. And even with explosions and some bigger battles (compared to Skyrim), I still don't get stutters or much frame drops either. I can spawn in 30 enemies in Skyrim, and performance drops so much (even on a 100% vanilla game). I do the same in Fallout 4, even as explosions and gun fire and everything is going off...and performance is still really great with no or very minimal stutters, and that is even with mods still left on.
I still kinda like Skyrim more as a game, but I'm enjoying the stability and lack of stutters as it loads things in WAY more in Fallout 4. Bethesda really made huge improvements in the engine they use in that regard.