OP, how would you manage your collections with PW4?
Have you thought of an Onyx or similar Android-based eink ereader, that can run Calibre Companion? I am looking into them but I'm not convinced by the startup time, which can be long on some of these devices. But then they can be left in standby for days, so maybe not a huge deal.
Or, alternatively, have you thought of having Calibre Companion on a phone, and then something like a Kobo as the main reader? This should allow you to browse search etc on your phone and, once you have identified the book you want to read, you'll easily find it on your Kobo. This of course assumes the Kobo is synced to your library.
I understand that Calibre companion supports hierarchies up to the 2nd level:
http://cc_faq.multipie.co.uk/index.p...=86&artlang=en
and that Kobo shelves can be created based on any column of your choice:
https://wiki.mobileread.com/wiki/Kob...nd_Collections