I bought the 39″ Seiki 4k display on a whim as it was only $389 at Amazon and Newegg, after reading a post from another programmer about using it as a coding display. My initial findings and notes:

• Make sure to have a snugly fitting Phillips head screwdriver before trying to mount the base.
• Plugged work rMBP into it via the suppled(!) HDMI cable into the HDMI 1 input on the of the display.
• There are only two scaling options for the rMBP: 3840×2160 (unscaled) and 1440×900 (not really worth the display if you’re going to do that.)
• Dial sharpness down to 0 to be able to read your fonts.
• Lower brightness below 50. I have it down to 24, but you may have a different preference.
• Color was a bit too vivid…  I reduced to around 25.
• I can divide my work window into quadrants and have full view of each:
  • Code window with 4 vertical and 2 horizontal split panes in MacVim.
  • iTerm window divided into 4 80×25 quadrants.
  • Browser window
  • Evernote window
• The internal speakers on the TV sound like a cheap pair of speakers. More output than the rMBP internal speakers, but sound quality is a bit worse.
• iTunes visualizer and YouTube videos don’t show the weakness in the 30Hz refresh rate, only a bit of higher speed mouse tracking has even remotely exposed it for me, but I’m don’t a desktop gamer.
• Mouse speed vs being able to see your mouse pointer in motion can be a bit of a challenge. The larger display is more conducive to keyboard input for me.
• Attempting to use the entire screen at once seems to introduce lag from eye-tracking, I’ve found it more useful to divide work areas on the screen. The benefit then comes from having everything on top at once.

Update:

I am now mostly using the settings listed in this article, except with a brightness of 35:

• Picture Mode: User
• Contrast: 38
• Brightness: 45
• Color: 36
• Sharpness: 0
• Color Temp: Warm