I feel there is a significant difference in the open world both games offer.
In The Witcher 3 it mainly serves as a background for different storylines to unfold. The main quest forced me to go on a nightly trek to Oxenfurt. I had to travel through lush forests, monster-infested swamps and bloodstained swamps. I could already see the lights of the city in the distance (such a beautiful sight) but I was denied access at the entrance. This is where The Witcher 3 shines: incorporating its beautiful open world into the story, giving quests the extra dimension of travel, exploration and sightseeing. It's the combination of meaningful storytelling and travel that elevates the open world to a higher level.
Fallout 4 on the other hand is very light on storytelling and the motivation to explore the world here is not storytelling but the exploration itself. Simply following quests painted the open world in a very disappointing light for me. In my first fifteen hours I was directed from one raider's camp to another, asking myself why I was doing this. There is very little cohesion, no internal consistency and it offers you very few opportunities to role-play. I can imagine that just exploring the open world and stumbling upon caves and secrets can be fun though.
To me The Witcher 3 feels like an actual medieval land and in comparison Fallout 4 would be a medieval-styled theme park zone. Both aim for different things but I vastly prefer what The Witcher 3 has done.
