I'd recommend the Epson EH TW8100 which I reckon you can get for around 2K now.
I have one and it's really, really good. with the TW3500 I had before it it was very good, but the dark detail could sometimes get lost when bright things were also in the scene, I could never quite get the pop I wanted in daylight conditions, and it seemd to periodically lose focus.
The TW8100 seems to suffer from none of these and produces truly fantastic pictures. The detail is amazing, black levels and detail excellent, and motion very good. With BluRay and HD TV pictures honestly sometimes virtually look real. It's also compliant with the Full HD 3D RF standard so I use £15 Samsung glasses and though I'm not a massive 3D fan it's very good.
Other bonuses are it has an automatic lens cover, 10 customiseabkle and renamable setting slots, good vertical and horizontal lens shift for easier placement. It even has decent frame interpolation, though I don't use it much.
It is a cracker for the price, and doesn't suffer from the rainbow effect as it's LCD. It's Epson's 2nd to top of range projector, but shares the same optics and image processing as their top model, the main omission in comparison being you can't fit an anamorphic lense.
It all depends on your priorities but I reckon it will be a good 3 - 5 years before we see lots of good 4K content, so I'd say instead of waiting and / or paying a premium for a 4K projector I'd get a 1080 projector now and get a 4K projector if you want one once the technology is established / prices drop.
Hope the above helps.