I suspect the vast majority of players have only played Fallout 3 and New Vegas, though I might be wrong.
How many of the 6 previous Fallout games have you actually played?
Do you expect there will be a sizeable audience for Fallout 4 of people who have played none of the past games?
* The two Brotherhood of Steel games may not be considered by Bethesda to be canon, but they happened so I've included them.