I agree. 1080p on a 27" or even a 24" is pretty bad, but on a 14" is essentially "retina" at normal working distances.
Certainly a 4K would be somewhat smoother, but at the cost of battery life, the need to fiddle with display scaling, etc. I just don't see it being worth it. 2K at most for a display of that size.
Now, a 4K at 32" would be a nice external monitor, and would give plenty of screen real estate (and yet would have a lower PPI than a 1080p at 14").