It will absolutely stay ahead. End of PS4 holiday 2016 works out as end of April 2020 for Switch.
PS4 - 53.5m (December 2016)
Switch - 54.5m (April 2020) (Prediction)
Switch will remain ahead on launch aligned for a long time possibly even until 2023.
Yeah, I'm expecting the Switch to stay ahead, too. But PS4 will then have January through March, which are generally higher in sales compared to Spring/early summer, so PS4 might pass the Switch again, or at least close the gap until the next holiday season. After all, 2017 was the PS4's best year, so this may very well become a very close duel next year, even if the Switch sells 20m+ then, as the brunt of the difference should come from December, which Nintendo generally dominates.
If he Switch can stay ahead throughout next year, then it will also very likely outsell the PS4 unless it crashes like the Wii after 2010 - but I don't see any reason why that crash would happen right now.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454