Good, see you in about 3-4 years then.
Switch sold over 1.4M in the US YTD but the PS4 less than 1M. While the PS4 sales have dropped, even in it's best year it couldn't have kept up with the Switch sales from this year. And the Switch is still at full price, so there's still much space for pricecuts to boost sales later on, ensuring that the Switch will have a long shelf life, or at least longer than many believe.
In other words, I'm pretty sure that the Switch will outsell the PS4's best years in the US during the next years, making the advantage melt down fast. How long exactly it will take depends on how well and long the PS4 will sell from here on out, but if the PS5 really has backwards compatibilty out of the box, there's not much reason anymore to buy the console (at least not a new one) after the PS5 is out. So I expect the PS4 sales will drop fast after the PS5 is out, much faster than the PS2 and PS3 did.
This is most likely going to be the Switch's best year, with the launch of new HW, possibly with a price cut, and Pokemon. That will be pretty much all of Nintendo's big guns fired. Maybe one more price cut. PS4 may not have too many big games launching this year, but it does have room for price cuts and may have another HW revision, as well. It will also see many cross-gen releases that will be on the PS5, as well, in the coming years. And considering the PS5 will most likely be $449-$499, budget gamers will be more inclined to buy a PS4 at $199, maybe eventually $149, to play those games. Nothing that will bring it back to its former glory years, but it will add to its legs.
Right now, the PS4 has a ~17.5M lead. It's going to take much more than a 2M-3M lead in its best year to make that kind of gap up.