Another reason to wait is that in order for a TV to upscale from 1080p to 4k, it has to interpolate the pixels. This requires a lot of processing power, especially with the number of pixels and the refresh rate of 120Hz. If you get a lower end 4k TV, the upscaled image could very well be worse than on a regular 1080p TV.
When you go to a retail store to look at TVs, they only send 4k signals to 4k TVs, to avoid this being apparent. They also show slow moving images, because lower end TVs can suffer from appearing blurred with fast moving images.
This is, IMHO, another good reason to wait for the technology to catch up.