It's really VERY simple - it won't play at all!.
A BD disc can't store enough data for a 4K film, so 4K BD's are totally new technology, requiring a completely different player to read them - just as a DVD player can't read BD discs, or a CD player can't read DVD's (or BD's). I'm presuming, just as with all the other disc technology changes, it requires a completely different laser (thinner beam) to read the disc.
The 'mistake', which causes the confusion, was calling them 'BluRay' at all.