There is no feasible method to create a portable device for an economically viable price that would be able to play PS4 games.
You would still need a TDP of around 10-15W to reach PS4 power these days. This would mean that a handheld PS4 would have to be around 50% thicker and twice as large as the Switch just to have a similar battery life. And with such a battery, the device would weight close to 2kg, way too heavy for extended use as a handheld.
@OP: PS4 did lose a bit of steam, just compare the weekly global sales from last year and this year. It got an early lead of 196k with MHW but then steadily lost almost every week to land at 230k below last year's sales 10 weeks later.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/