I have not purchased Fallout 4 still, but some PC gamers are saying that the PC version of Fallout 4 is the most optimized Bethesda Game Studios developed video game on PC.
They say there are less stutterings and Crashes to Desktop (CTD's) than what the PC version of The Elder Scrolls V: Skyrim had.
According to Gstaff and some PC gamers who are modders the PC version of Fallout 4 has a 64-bit executable and uses DirectX 11 and has Physically Based Rendering (PBR).
There is also a rumor on a AMD Radeon video card picture that I saw I think a day after Fallout 4 released for sale that Fallout 4 will get DirectX 12 support.
DirectX 12 allows for a lot more drawcalls and when I say a lot more I mean A LOT more.
DirectX 12 allows for 300,000+ drawcalls, 500,000+ drawcalls, 3+ million drawcalls and I heard Microsoft wants to try to reach 50+ million drawcalls with DirectX 12.