With Switch's line-up this year looking so promising for the Japanese market, I'm going to chance it and say Switch could do 4.5 million within Japan in 2019.

With such a crazy high number it would definitely need a price-cut to do it. However if it doesn't and it still manages to come even close at around 4 million, that would still be incredible for the Nintendo Switch in my opinion. Here's hoping, I think Switch deserves it.

Nintendo Switch:

4.5 million - with price cut
4 million - without price cut

PS4:

1.6 million

Switch 4.5 million
PS4 1.4 Million

That looks about right to me too.

Switch 4.5m
PS4 1.4m

Switch : 4.5M( Mainline Pokemon, Animal Crossing, FE three houses, DQ11S and hardware revision)
PS4 : 1.5m( Aggressive price cut and free games promotion during the holiday period )

You beat me to it, I'm expecting 4m for Switch & 1.5m for PS4.

PS4 1.5m
PS5 launch in 2020. Personally I'd prefer 21.

Switch 3,9 Million
PS4 1,45 Million

Switch: 4.35M

PS4: 1.35M

I'm pretty sure the Switch'es baseline will be several K above last year, pretty much not dropping below 40K and half the time selling above 50K, with Pokemon giving a similar boost to holiday sales (less high, but spread over more weeks) as did Smash.

For the PS4, I fear the baseline will drop further and a steep drop incoming 1-2 weeks after KH3 release. PS4 won't get nearly the same boost with KH3 as it got last year with MHW and an even lower baseline means the lines spread out more and more.

NS:3.5m
PS4: 1.3m

Switch: 4.5m
PS4: 1.35m

With lineup Switch is getting this year, very possible price cut and revision, I don't see possibility Switch selling less than 4m in worst case.