A PS4 Portable while possible, does seem kind of unlikely. When PS said they had to pay attention to Switch, I don't think they necessarily meant they need to build one. Trying to out-price it like they did with XB1 would make just about as much sense as trying to create a mobile PS4.
Yes, some gamers want the mobility, but there are plenty of others who simply require a cheaper stationary PS4 to be able to enter the ecosystem. Like some others have said, the cost and size at this point in time would most likely be a problem, unless PS wants to just get anything out on the market and is also willing to subsidize it.
What would be more likely, would be a PS4 super slim and Pro slim. It's been two years already since PS4 slim and Pro launched, and if super slim and Pro slim launched this holiday, then if PS5 didn't launch until late 2020, you could possibly end up with a PS4 super duper slim or Pro super slim late 2020 as well for super cheap, maybe. If PS5 is launching late 2019, then getting the PS4 super slim and Pro slim out now would almost be vital.
The one way I can see a PS4 Portable launching this holiday, is if PS and Nvidia have been working together for some time and have a custom Tegra of their own. Getting third parties to port to Switch hasn't exactly been easy, but if PS has an extremely similar device, then that very well may change things. While I also think this is unlikely, both PS4 and XB1 ended up with very similar hardware in the dedicated home console space, so who's to say that PS and Nin couldn't on the handheld hybrid end?
There is the slightest possibility that AMD has an ARM based APU they've been working on for PS, but I would say that's much less likely than Nvidia getting PS on board with Tegra. Otherwise a 7nm Ryzen based APU that's BC with PS4 games should definitely work for a portable, but that wouldn't be until late 2019 at the earliest due to manufacturing process yields/cost.
PS1 - ! - We must build a console that can alert our enemies.
PS2 - @- We must build a console that offers online living room gaming.
PS3 - #- We must build a console that’s powerful, social, costs and does everything.
PS4 - $- We must build a console that’s affordable, charges for services, and pumps out exclusives.
PRO -%-We must build a console that's VR ready, checkerboard upscales, and sells but a fraction of the money printer.
PS5 - ^ -We must build a console that’s a generational cross product, with RT lighting, and price hiking.
PRO -&- We must build a console that Super Res upscales and continues the cost increases.
