Never got around to producing a working 4.0 release (at least yet). Interest in Calibre2opds seems much lower so not a huge incentive to work on it
The main feature I have done some work on is support for custom columns in expressions used to filter catalogues. The other work has been general work On updating components used internally.
A feature that someone raised that is theoretically possible and would be beneficial is doing the xml transforms on the client side (which most current browsers should support) to avoid the need to generate separate html files.