According to this poster at blu-ray.com, Target’s already got individual 4K titles in their system already, and they’re dated for release simultaneous with the box-set (March 31st), which appears to still be a Best Buy exclusive.
According to another poster, the individual 4K titles are 3-discs each (4K/Blu/Bonus), at least in Germany and Australia. That might hold true for every other region too, although people are still worrying/“hearing” that individual titles in NA won’t have the bonus discs, and the only way to get them would be through the box-set.
it’s appearing as if the only upside to getting the box-set is cosmetic, as none of the on-disc content will be exclusive to it, so if you don’t care about the box, the letter, and the special book/artwork, you’re probably okay simply buying the movies you want individually. I believe this is the first time in quite awhile that Lucasfilm is allowing you to essentially part out a brand new set at retail. In 2004 you couldn’t just buy individual DVD titles in the OT, you had to get the full set or nothing. Same with the 2011 BD set (although I believe shortly afterwards they let you buy either trilogy in a separate set, sans bonus discs). But for 4K, it looks like they’re saying “you don’t HAVE to buy a box to get all the goodies. Just pick the movies you want.”