804 D3 owner here. I wouldn't expect a day-and-night improvement with the D4 series. Personally, I'm smitten by the new 801 D4. Just love it as an object d'art.
Not that crazy about how the tweeter sits on the midrange "turbine", but should look OK from a head-on perspective. Hope B&W shows at Axpona this year (hint: 801s with a pair of those McIntosh MC901 Hybrid amps!).