First the good news, my Verbatim/ Pioneer BPR-UD04 is currently processing the disc successfully.
However, for the second time my LG WH14NS40 declined to recognise the disc, this time declining to recognise its very existence.
However, it did recognise my 4K Disclosure disc. After a slow start it rose to 4.3x and then fell to 2.0 halfway through.
I suspect that the Verbatim/ Pioneer drive would have chomped through at 4.0x throughout.
So, my LG is still reading 4K discs but it is a bit picky about which ones.