You do realize that, as of two fiscal quarters, PS4 tracks ahead of every console ever, including PS2 and Nintendo DS? That's hardly a metric for gauging how well Switch will sell versus PS4 in the long run. PS4 had the absolute strongest start of any console, but it never quite reached the heights of PS2 or even PS1 peak sales (22.5 and 21.6 million in a year, respectively,) let alone the peaks of DS or Wii (31.2 and 26.0 million).
I'm not saying that Switch will do DS numbers, but judging its long-term potential based on PS4's launch sales is absurd.
Why are you talking in past tense, like PS4 is already over and done? It hasn't even finished it's fourth year on the market, and we still don't know if this will in fact be it's peak year; people are simply assuming that based off of past sales trends. It's still tracking ahead of PS2 in aligned Charts, so we will see how it does in the long run. I doubt it will stay ahead of PS2, but I still see it doing 130M+ in its lifetime.