That would be true if everything stays at the current speed.
But while Switch should accelerate during Summer and boost in Fall with all the announced game releases during that period, and then go into overdrive during the holiday season with Pokemon and probably Animal Crossing this year, PS4 will mostly slowly slide down in sales, even with a 50$ pricecut, and just boost a lot for Black Friday.
In the end, the difference should be more around 5M, if not even more than that.
Only Cyberpunk could exponentially increase PS4 sales like RDR2 did. Even with TLOU2, DS and GOT all releasing in 2019 it's hard to believe that it will put a fight to the Switch unless there is a PS4 Pro price cut.