By the time the Switch could even catch up to the PS4's sales (and that's if it does), the PS5 will probably have already been out for a year or two. I see the PS5 as a 2019 release, 2020 at the latest.
Still, the Switch is doing really well so far.

Lifetime Sales Predictions 

PS4: 130 mil (was 100 million) Xbox One: 55 mil (was 50 mil) Switch: 110 million (was 73, then 96 million)

3DS: 75.5 mil (was 73, then 77 million)

"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima