I'm going to throw out a little different perspective. Every day that passes we get closer to knowing what the "true" 4K specs will be, and the availability of content (ie a 4K Blu-ray Disc). For a while it looked like this holiday season, but now it's looking like a 2015 or possibly 2016 phenomenon. On the one hand, now is a great time to buy as the quality has never been higher and prices never lower, but at the same time, if you've held out this long with a 720p machine, or maybe more importantly, if you don't want to be "out of date" in a year or two, then you may want to hold off buying anything until you can get something you know will fully support 4K media.
Right now the only 4K machines available (Sony's) don't support what seems most likely to be required by 4K media, ie high bit depths (10 or 12 bit color), higher color subsampling (ie 4:2:2 vs 4:2:0), or higher frame rates (ie 60fps). Personally if I were going to spend $10-20k on a projector today, I would not want to feel like I need to replace it in a year or two when 4K BD comes out and it's features/requirements are not supported.
On the other hand, if you were to go with something like a JVC RS4910, or similar, something "inexpensive" but that throws and awesome 1080p picture to tide you over, that might be a good option.