From Canon USA:
Real 4K technology goes far beyond the label. Canon 4K optics are born from decades of research and design, creating brilliant, vivid, rich images. And it all begins with the lens.
Note: The factors outlined in this video – resolving power, contrast ratio, lateral chromatic aberration & modulation transfer function – are applicable to all lenses, not just 4K ones. On a side note, isn't it kind of ironic that a movie explaining the aspects of 4K lenses is only available in 720p?