Samsung is showing new 4K OLED TVs prototypes at the IFA 2013 exhibition. Samsung calls these panels a 'proof of concept' - which probably means we shouldn't be expecting this panel in a commercial product soon. They are showing two models, one flat and one curved.
It's still not clear whether 4K at a 55" TV actually makes a lot of sense, but it seems that this is the way the market is moving, and it's good to see more 4K OLED TV prototypes.