With the HX850 gone... which I hesitated and missed, I'll be going with a w900a if the reviews don't rip it apart.
I'm sort of excited by the Triluminous quantum dots and the use of Rec.2020 (UHD) colorspace. I suspect Sony will be using that to enhance BD playback.
Sony has said that they may not be looking for a software format for 4k movies. (Probably just streaming and PS4 stuff.)
Instead they are favoring a Superbit like mastering process where they start with a 4k master and specially compress it to 1080p on Blu-Ray.
All BD players will be able to play Sony mastered BDs, but Sony BD players with Sony 4k TVs will produce a better quality upscale than from a standard BD.
While I believe 4k to be a placebo, the w900a will be using the same expanded color gamut and triluminous display as their 4k televisions.
One could easily assume a Sony BD using the above method will deliver Rec.2020 color expansion on the w900a.
Expanded colorspace might be a more pronounced improvement to TV than 4k on a 55" screen you would need to sit less than 6 feet from.
That's my prediction anyway.
@Browninggold, as said on some new sites, the HX950 will continue to be the standard HD XBR TV from Sony until the fall. At that point expect a w1000a or something that will incorporate the tech and features that are new to the w900a.