For me, my player does not play 4K discs I do have the Criterion Blu ray release and it was stunning!! I bougt it the same time I bought "The Life and Times of Cornel Blimp"another classic UK production.
Newer films benfit from the 4K format, with older films ( pre 70) I have seen little difference between Blu-ray remastered and the released 4K versions, but that's just me...bottom line is if the film can benfit from a digital remastered presentation in either an upgraded Blu-ray or 4K edition...