I definitely believe it would be possible by late 2019. If I'm not mistaken, AMD is releasing the successor to Raven Ridge in 2019, their mobile APU line. You'd be looking at a low power APU (the current Raven Ridge APU's range between 12 and 25 watts) with Ryzen 2 CPU cores and Navi GPU cores. Then all Sony would have to do is underclock it or disable some cores to reduce power consumption so they could use desktop GDDR5 instead of LPDDR4, and make it larger than Switch so it could have a bigger battery, and they'd have a system comfortably able to play PS4's library (after patching) at reduced resolutions with good battery life.
Like DF stated, PS4 for 720p games could be possible in few years but form factor of such device would be similar to laptop, and than every single game would need a quite of rework to run at 720p and at weaker hardware (definitely some 3rd parties wouldn't want to do that), so all that dont sounds attractive at all so hardly that Sony would try do something like that.