Quote - I've been using V4 for a while now but I've never been very happy with V4's lack of facial morphs and 'plastic' eyes.  Looking through my runtime I noticed under my Daz people that I have a character 'V4.2 to V3'  While loading it I get a message "Please locate "V4toV3BODYGrps.pz2".  Because P8 can't find the file, the program then crashes.

I've done a complete file search but can't locate it.  Where is it?

If I can then load this character, does it allow V3 facial morphs to be used with V4 textures (my preferred scenario)?

Also, in the disgraceful absence of any documentation from Daz on V4's morphs and character types, is there any tutorial that covers this?

That "disgraceful lack of documentation" appears to be the norm rather than the exception for a fair few key Poser / Daz products. Some vendors provide excellent documentation, so it's obviously not impossible. Unfortunately, when you're trying to unscramble Shader Mixer brick behavior without documentation, you're going to find yourself at a distinct disadvantage.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks