Microsoft Store offers the Dell 28" 4K LED-Backlit LCD Monitor, model no. P2815Q, for $299 with free shipping. Although yesterday we saw it for $25 less, that's the lowest total price we could find by $37, outside the price below. It features a 3840x2160 native resolution (four times that of 1080p), LED backlight, and HDMI input.
A close price: Dell Small Business offers it, bundled with a $150 Dell eGift Card, for $449.99 with free shipping.