I also went to the Starpower Expo so here is my opinion on the sets, for what it is worth.
I viewed both sets and while I did notice a better picture than the 1080p displays, I can't say I saw anything that blew me away. I should note that it was very busy while I was there so I was not able to determine the resolution of the content on the Sony set (the Sony rep was surrounded by people). I did spend about five minutes sitting in front of the Sony from a distance of about 6-8 feet. It looked very crisp, showing content off the PC attached to the TV, of a city scene. As Kamoflage mentioned as well, I did not notice any pixels even at VERY close distances. All that being said, I would not consider the 4k technology to have enough wow factor at this point to make a jump (assuming I had that amount of coin to spend, which I certainly do not).
The LG display was equally as crowded. It also looked crisp, though I was not able to sit in front of this TV like the Sony.
I will say the Sony 4K projector showing Open Range in the JBL Synthesis room looked VERY nice. Definitely worth a trip to see that projector and the Synthesis system in action. Very Cool! (I know, off topic but I had to mention it).
Point being, for me anyways, I only saw a minor incremental quality improvement when looking at the displays from normal home viewing differences. So for now, pretty to look at, but not where my money would go if I had the means to purchase in this category.