there are current TV shows that will never be 4k, older ones have got no chance!
This is a common misconception.
Current shows are shot on video, if that video is 1920x1080, it will never be anything more than that, whereas film has a far higher potential. So in theory, anything shot on film (like older TV shows) will gain from a higher resolution scan than plain old 1920x1080.
In reality it will probably be only a marginal improvement, given that the film stock used for TV, - although far better in resolution than was necessary - didn't need the fine detail cinema productions did. Nor did most for-TV shot material contain the amount of detail motion pictures do.