Well it's between Sony and Nintendo. It comes down to what big games are coming and what deals will there be. If Smash is the only big thing Nintendo has Sony will probably win. If Nintendo also gets some big third party games (even just really big ports) and one more major Nintendo franchise it definitely could win. If Sony drops the price (which they probably will this year) and Nintendo doesn't get aggressive on holiday deals then Sony will win.
My guess is Sony will have a huge lineup and a price drop at some point this year, and Nintendo will have a lot more for the Fall than what we know about, and if they are trying to ship 20 million this year (or fiscal year) they are gonna want to get very aggressive with holiday deals to sell what they ship (might actually see Switch on sale for Black friday for instance) in which case I could definitely see the Switch taking it.
In general Switch is obviously on the rise and PS4 is likely just at the start of its decline, though perhaps the decline won't really start until after this holiday season.