Depends which department worked on the game first. Take InXile as example. I think, it was Fargo who said in an interview, one of the reasons why they started to work on Numenera before Wasteland 2 was finished was to give the concept artist something to do. Otherwise, they would had to either dismiss them, or have a whole team doing nothing. Which isn't really ideal.
So, who knows? Bethesda might had even started to work on Fallout 4 long before Skyrim was released. But maybe it was a team of 2 or 3 people which got bigger over time, with more and more teams assiged to work on F4, the moment they finished all of their work on Skyrim and its DLCs.
However, I doubt more than 3 years have been spend on Fallout 4 in full production, meaning, all teams beeing involved in the development. The game simply feels to unfinished for anything that took more than 3 or even 4 years of development. Leave alone 6 or 7 - Who's going to believe that anyway?
I mean yeah ... you could always say they made a lot of mistakes, had to cut a lot of stuff, starting from scratch again etc. Duke Nukem style. Kinda. But does that sound really much better from a team of supposedly professionals?
So, in other words they either outright lied. Or they used their propaganda tools and playing with words. 2 people working on Falout concepts during Skyrim, is not full development in my opinion. Or they are plane and simple incompetent.