The general belief is that 4K will come with h.265 which is billed as a much more efficient storage format than the current h.264. File sizes won't necessarily be that much larger for 4K if h.265 is used. Apparently, Apple has already at least somewhat embraced h.265 in FaceTime software on iDevices.
BUT, for those consumers who don't want 4K video files, no problem: just as those who purchased

TV3 could opt for the 720p or even the SD version of a video, I would expect a 4K

TV to also offer the 1080p, 720p or SD versions too. Anyone with data issues or "too slow" broadband could simply keep doing what they are doing now. Better hardware will have no trouble playing lessor software.