If you must have a 4k now, I would recommend either the Samsung UN55HU9000FXZA or the Sony XBR55X850A (both also come in bigger sizes as well)
However I would say don't get a 4k just yet. not enough places are broadcasting in it yet, & the motion refresh rates on them is not all there yet. You still do get some blur during faster motion scenes. i would wait a few more years for them & the oleds to drop in price. OLED is supposedly better, richer & more realistic color.
If you choose to go with a non-4K TV, I would recommend the Samsung UN55H8000AFXZA or the Vizio M551d-A2R. The Samsung will definitely be better picture quality, but value wise the Vizio M-Series is one of the best on the market.