Well that was a trauma of sorts!
First time in a long time that a minor point upgrade has gone skew-whiff.
Though I used system definition 14,1 and don't use Lilo etc I got a black screen after boot.
Okay, I forgot to tick the "Install to ESP" option when I upgraded Clover to the latest, as advised. My mistake.
In this situation I discovered even more of the intricacies of using Terminal to rescue a system than I wanted to. I also discovered that quite a few standard Linux/Unix commands were no longer working. (Try "unzip" for example). Nothing would re-install Clover from Terminal (that I could manage).
So I had to restore 10.12.3 - which was as rock-solid as before.
Then I re-installed Clover with the correct tick-box and the mounted EFI partition open before me so I could see what was happening. All went fine so I rebooted and then took the plunge and installed the Combo update to 10.12.4 again.
Everything is as it should be, all working once more. Networks, sound and audio.
Surprised we needed a whole new Clover install just for a point update. Anyone know what changed so dramatically from 10.12.3 to 10.12.4 beyond the bug-fixes and NightShift?