I am an amateur, so just to share some casual experience.
I suspect it has to do with the 4k TV internal decoding/upscaling. Almost all 4k TV I have seen in demonstration perform poorly when showing regular TV signals, even worse than 1080p models in same sizes. Most don't even perform well when playing BD. The only time I see great result was a Sony 4K TV playing BD using the latest Sony BD player. I heard from others that the same Sony 4K TV performed poorly when using other brands of BD player. Therefore, I suspect poor images from regular broadcast TV may be due to similar underlying decoding/upscaling issues.
I am tempted to buy Sony 4K TV but holding on until:
a. 4K BD player becomes available
b. 70inch+ model, makes no sense to buy 4k for screen smaller than 60"
c. Broadcasting TV quality improves