The shoe doesn't promise 4% off of your PR, it is only claiming to provide a 4% increase in efficiency. This means that it will be 4% easier to run your PR time. There may be a correlation between an increase in efficiency and faster times, but there is by no means any certainty to this, so I don't think Nike is actually claiming that you will run 4% faster.
The curved carbon fiber plate inside the shoe will apparently put less strain on the legs (due to the increased efficiency). The shoe is obviously not fine tuned to the average person's stride, but I find it likely that the shoe will offer an excellent ride and MIGHT lead to an increase in performance. Either way, I am itching to get my hands on a pair and see what they can do. I don't spend a huge amount of money on running shoes to begin with, so while the $250 may set me back a bit, if these really are a great innovative pair of shoes I think it may be worth it.