Here is a discussion about how the sqlite4 experiment ended, from 8 months ago

https://news.ycombinator.com/item?id=15648280 (157 comments)

And the relevant commit (timestamp 2017-10-27):

https://sqlite.org/src4/artifact/56683d66cbd41c2e

> All development work on SQLite4 has ended. The experiment has concluded.

> Lessons learned from SQLite4 have been folded into SQLite3 which continues to be actively maintained and developed. This repository exists as an historical record. There are no plans at this time to resume development of SQLite4.