CPO brought out three volumes of his quartets to wit
Op.70 Nos. 1 & 2
Op.126 No.2
Op.150 Nos. 1 & 2 and
WoO 10, 37 and 48
Does anyone know is there a Vol.4 or is one forthcoming? And, have any of his other string quartets been recorded elsewhere or on radio broadcast?
Per Schuppanzigh-Quartett.de (https://schuppanzigh-quartett.de/stationen/) volume 5 is expected in March. (??!) (aufnahme?) Maybe a typo for iv- might be worth emailing them (well, I assume you have done!)
and yes, op.166/2 was broadcast on Radio 3 performed by the Bochmann Quartet (1986-07-03, says CADENSA.) WoO 37 may also have gotten a broadcast performance in but CADENSA just went unavailable (at least to me)... ah well.
As Eric helpfully informs us, vol.5 is scheduled for recording on 5th March 2021, although no mention is made of its contents.
I imagine vol.4 has already been recorded and is awaiting release.