Besides obvious similarities the games are different in many aspects and have tottaly different purposes and BTW you are also wrong stating you can compare it to SKYRIM because they are in the same genre... nope.
Witcher series have always been about the story, many people didnt wanted them to make them open world but they still did it and while they did an amazing job with that game it seriously underwhelming to explore, there is literally no incentive or purpose to do it, when you play TW3 you are there to conclude Geralts story and thats it, the side quest are also amazing because they are also story driven so like i say you are there for the story for the writing, not to explore something hidden etc because the incentive is minimal, the map for starters points to you every damn place where you can find loot, like... WTF???? that alone eliminates all the purpose of exploring, now the second problem is most of the time you only find [censored] that serves no purpose specially when you start crafting your own equipment...
You are there as Geralt a Monster Hunter that has a purpose, has a story already, has everything already you can just change minor things but not the overal picture and the game focuses HEAVILY on that aspect.
Skyrim and Fallout are more about freedom and exploring and of course the mods, Skyrim did a poor job on the loot though because eveybody knows how OP is to craft your stuff in that game, so again that hurts the overal incentive to explore, something that is fixed in FO4 with the elite mobs, also no Bethesda game marks you in the map where is the loot you need to explore and find stuff yourself, Skyrim main story was weak and just there because... it was not really that important and you can just ignore completely i dont know how better the story is in FO4 since im not far into the game because i have like 8 hours played and most of that time was exploring and messing with the settler stuff and finding stuff to improve my settlement, something i didnt feel like doing in TW3 because there is really no point.
TW3 graphics are better period (even after the downgrade), but FO4 graphics are not bad or terrible as some people like to say, game looks good and thats it.
Complete diferent games like there is no point in even try to compare gameplay because their purposes are different.
Only OFFENSE Bethesda did was the TERRIBLE controls and UI for PC, like easily the worst of the year and probably one of the worst ports ever in that aspect, so they better get their [censored] together and fix that soon.