However, the Switch has a 6 million lead over the PS4 with one Christmas less for the Switch, right?
It is impossible for sales to fall.
Nothing is impossible, you can't know what'll happen in the future. Surely no one imagined Wii sales would decline like they did. Not saying the Switch will fall off a cliff like that, I don't think it will, but it is definitely possible that sales will slow down enough for it to not reach the PS4. With Sony consoles, all of them have had pretty similar sales curves, so it's easier to make a prediction based on those, but Nintendo consoles aren't as consistent. The handhelds are, I suppose, but it remains to be seen if a hybrid console like the Switch can continue selling for years when up against renewed competition. This is pretty much an unprecedented situation in the console market.