Fallout 4 development began with Skyrim port for XONE

ATTF: "Fallout 4 began development long before the PS4 and Xbox One had their final hardware figured out. This meant that once it was, the team at Bethesda has to become familiar with the systems so they could get their game running. To do this, according to an interview with Game Informer, the team ported The Elder Scrolls V: Skyrim to the Xbox One."