My Surface Book is 267ppi (3000×2000, 13.5″). It’s very good, but at typical viewing distances it’s still unquestionably lower than the human eye is capable of resolving. I doubt there’s much value in going higher, but it’s not useless, either—if all else was equal, 400ppi would certainly be more pleasant to look upon than 267ppi.

4K on 17″ is 260ppi, which I reckon is pretty good. On 13″ or 14″, full 4K is probably a little bit of overkill at present (315–340ppi), but 1440p is definitely less than I’d like even on a 13″ monitor (226ppi).

Of course, it’s always a matter of balance, because all else is not equal; higher resolution means higher cost, higher power consumption, higher memory usage, and higher processing requirements. But I’d definitely prefer 4K to 1440p even at 13″.