PS4 had an amazing 2018.  Only 1 week below 200k and even that was 199,103.  Few consoles have ever done that (Wii, PS2, GBA and DS are the only ones I can image having done it..unless the X360 also did it with Kinect).

However, Nintendo has always been a second half sales player whereas Sony tends to be much more all year rounded.  While it looks surprising, it's actually not when you look at the year long trends of the console makers every year.

Look at 2018 first half for both:

PS4 - 7,016,919
Switch - 4,898,946

Now look at the second half of 2018 for both:

PS4 - 10,983,848
Switch - 11,587,952

So despite the 2.1 million deficit at the half way mark, Switch finished 600k ahead in the second half. So it may seem odd that PS4 H1 2018 is a million higher than Switch H1 2019, it's not beyond expected figures given how they sell across both halves of the year.

While the PS4 is in front, unless the announcement of the Switch Lite throws a wrench into the Switch's sales wheels until it's release, The advantage should start to shrink now over the course of summer, and then shrink rapidly/grow in favor of the Switch once the Lite is released.

Like SpokenTruth already explained, 2H from Nintendo beat the one of the PS4. Nintendo being traditionally dtrong during the holiday season coupled with the 1-2 punch of Pokemon Let's go and Smash Ultimate certainly helped to that, but it wasn't the only factor. Nintendo already announced back then that during Summer the sales went up, so the rise came from across the entire 2H last year. With what has been announced to come this July and then starting August 30, there will be a similar push this year throughout summer, and then Lite, Link's Awakening and Pokemon will push it into overdrive.

That's a cool interesting point. If it holds the same percentage increase then H2 2019 will sell 2.365 times more in H2 giving a total of 20.4 million sold for 2019. Which would give it a 2.4 million lead. So we'll see if that trend continues in the same way. The Switch should win, it's just without doing background it was interesting to see PS4 has a million lead in it's 5th year over Switch this year. 

Do you think Switch Lite would be tallied up with Switch sales? It's not as if they're just selling the switch controller without the dock. It doesn't support any output to a TV, no joycons etc, so it's a dedicated handheld device that will over time I assume replace the 3DS. 

It makes no meaningful difference either way but interesting to see if people would add those sales to the Switch.

That's a bit surprising, but I think it'll be pretty close in the end. Given that Switch is over a million ahead of where it was last year, it probably won't lose that lead on its previous year, and that makes it on track to sell 17,565,922 at the moment for 2019. Given that Nintendo's sales are holiday heavy, that lead will probably increase a bit in the end, so Switch will inevitably close much of that gap with PS4's 2018, and might even surpass it in the end. Clearly barring a disaster Switch's 2019 will beat its 2018, so the Switch 2019 to PS4 2018 race will probably be more interesting to watch.

Now that we're halfway through the year, let's do some math.

At this point last year, the PlayStation 4, Xbox One, and Nintendo Switch sold through 38.9812223%, 32.06302907%, and 29.71417668% of their final 2018 totals respectively.
If those same percentages hold for this year, these would be the projected final yearly totals for all three systems in 2019:

PlayStation 4 - 13,444,532
Xbox One - 4,922,161
Nintendo Switch - 20,424,830

Sony needs to give the PS4 a price cut NOW! RIGHT NOW! There's nothing Microsoft can do for the Xbox One that will make it's situation any better. And the Switch Lite just threw a monkey wrench into how the Switch will turn out, there's no telling at the moment if it'll be lower or higher than the number I have projected here, that's the most interesting thing to watch for.