I'd much rather have a top notch 2560x1600 screen than a mediocre 4K screen, when we're talking about the 13" size. Sure, a top notch 4K screen can be even better, but then you have to consider battery life and GPU as well.
Why do I say this? Cuz at 2560x1600, a 13" screen looks awesome at normal seating distances.
For my 12" MacBook, I have NEVER once thought, "Geez, I wish Apple would make this 4K". The 2304x1440p resolution is excellent. What I want on this class of machine is a second USB-C port, USB-C Gen 2 10 Gbps, Thunderbolt 3, a better trackpad, and a better iSight camera. And more travel on the keyboard would be great too.
A higher resolution screen isn't even on the radar.