4K is a long way off from being mainstream. If you shoot in HD, your fine. Shooting in 4K is nice, but you'll have to get software to take it down to HD quality for general circulation.
Only film/TV studios are shooting in 4K. It'll be a long time before we have 4K TV's in our homes, as the cable companies only recently upgraded their systems to handle HD.
This is what I know from a friend who shoot's movies in the "industry."