Thanks for that John. Another nail in the coffin for self burnt discs.
I wonder though whether 4K files burnt to a Blu-ray disc solely as files (data disc) will be recognised and played by a 4K Blu-ray player. Or whether it will even play 4K files from a USB thumbdrive? Unfortunately my Blu-ray player is 1080 only, as is my TV, so I cannot check myself.