If you mean a literal portable PS4, that would be virtually impossible, even the newer slim PS4 model still consumes like 63 watts, which is 7 times too high of a power consumption.
If you are talking about a portable device built with a new chips from mobile tech, yes by 2020 something in the realm of a PS4 would be feasible.
But you would be starting from 0 in software library as no PS4 game would natively run on that completely new architecture.
The other problem you would run into is Nintendo would very likely respond as Swtich is their main hardware line left.
And Nvidia could give Nintendo a chip of similar/better power than whatever Sony could get. And that chip would also be compatible with (by then) hundreds of Switch games too.
So it would be very easy for Nintendo to counter it.
