Walmart offers the Vizio SmartCast M-Series 54.6" 4K HDR Flat LED Ultra HD Smart Display, model no. M55-E0, for $398 with free shipping. That's $30 under our mention from six days ago ($50 under our Black Friday mention), $300 off list, and the lowest price we've seen. Features include:

3840x2160 (4K) native resolution

802.11ac wireless

SmartCast OS with Chromecast (with access to Netflix, Hulu, Amazon Video, more)

USB port & 4 HDMI inputs

remote control via mobile device with the SmartCast app