HDTV only really took off when there was more content than sports to make it worth while. Once you had it across most channels and on streaming and physical media sources along with HDTV panels becoming cheap it no longer made sense to not own an HDTV.
4K or marketing speak "UltraHD" will need lots of content and low prices.
To me though the biggest gain if 4k becomes popular will be lots of higher resolution computer screens becoming the norm. 1080p is the norm in computing because those are the cheapest panels right now.
The reason it isn't officially called 4k (or 4K if you want to get the units wrong) is that there is already a 4k movie standard - hence the correct term is UHD. I don't know why they didn't just call it 2160p so that everyone understood its relationship to current 1080p.