When you say it "takes a little while to get sharp" are we talking minutes or hours?
I uploaded a ~ 4 min clip @ 4k 30Hz in its original .MOV format about 30 minutes ago. I copied it from the SD card to my computer and played it back there with VLC and it looked great. The .MOV file was just under 1.8GB. My computer has a i7-4770k CPU, GTX 970 video card and 4k monitor running at 60Hz. My Internet connection is fiber and 100/20 Mbps. I can play other 4k YouTube videos that are 4k and they look stunning. It typically takes about 5-10 seconds before they "snap" into 4k mode.
The clip I uploaded will only playback at 360p (no other options available when clicking the cog in the lower right) and looks absolutely horrid on my 4k monitor. Here it is:
Any ideas?