SQLite4 is an alternative, not a replacement, for SQLite3. SQLite3 is not going away. SQLite3 and SQLite4 will be supported in parallel. The SQLite3 legacy will not be abandoned. SQLite3 will continue to be maintained and improved. But designers of new systems will now have the option to select SQLite4 instead of SQLite3 if desired.
SQLite4 strives to keep the best features of SQLite3 while addressing issues with SQLite3 that can not be fixed without breaking compatibility.
Very interesting, particularly the primary key and decimal stuff.