A PS4Portable would have to use an AMD SOC. That'd greatly affect pricing.
It's not just about the price of the SOC or device itself. It's what money will be brought in ? companies have moved past the ps4. Games aren't getting developed for it. Sony fans are saying devs wont even optimize to make games work on the xbox series s which is the same hardware as the series x but reduced in performance. Yet some how developers will start to program new games for a portable ps4 ?
So what would sony do ? Jack the ps4 games back up in price to drive revenue ?
At least a portable series s would get new games for another half decade or so if its some what successful. PS5 portable would get even longer term support.