Thanks again for your help. On the bright side, I think I’ve made a consistent reproducer, which I’ve “linked” below. To reproduce open that file, then add a new “Viola” section instrument, then immediately try to save the file. Hopefully Dorico will hang (and in %APPDATA%/Steinberg/Dorico 4 you’ll see an ever expanding log file).
https:// drive dot google dot com/file/d/15HLnDaIMPNMPd9VlVJSmI9aXsqgIZ4d1/view?usp=sharing