A 4-cog setup on BB has been an on/off, frustrating affair but now I may have resolved it!

An(other) inspection of the derailleur cable system confirmed that the cable had enough movement to suit 4 cogs but that the derailleur pusher wing-plate was restricting the rotation of the pusher, thereby causing inconsistency in shifting to the low cog (depending on cable tension, temperature, day of the week, etc?)

Some filing of the low cog wing-stop & all seems well?

Clarence & BB in their 9kg & 8kg 4speed guises