I do not know of any ongoing effort toward producing an EPUB 4 specification. I saw mention of it as a possibility a few years ago but nothing recently. I believe that the slow adoption of EPUB 3 and the lack of interest by publishers and book stores has killed the idea of developing a major change to EPUB for now.
EPUB 3.3 is in development and on track to become the next version of the EPUB specification. It is an incremental change to the current EPUB 3.2 spec. The
latest working draft was released on February 11, 2022. There is also ongoing work on
the EPUBCheck tool to test conformance of EPUB 3.3.