4.1.0 (October 12, 2019)

Potentially Breaking Changes

Technically, two changes are breaking changes, but in practice these are extremely unlikely to break existing code.

Conn.Begin and Conn.BeginTx return a Tx interface instead of the internal dbTx struct. This is necessary for the Conn.Begin method to signature as other methods that begin a transaction.
Add Conn() to Tx interface. This is necessary to allow code using a Tx to access the *Conn (and pgconn.PgConn) on which the Tx is executing.

Releasing a busy connection closes the connection instead of returning an unusable connection to the pool
Do not mutate config.Config.OnNotification in connect

v4 is a major release with many significant changes some of which are breaking changes. The most significant are included below.

Simplified establishing a connection with a connection string.
All potentially blocking operations now require a context.Context. The non-context aware functions have been removed.
OIDs are hard-coded for known types. This saves the query on connection.
Context cancellations while network activity is in progress is now always fatal. Previously, it was sometimes recoverable. This led to increased complexity in pgx itself and in application code.
Go modules are required.
Errors are now implemented in the Go 1.13 style.
Rows and Tx are now interfaces.
The connection pool as been decoupled from pgx and is now a separate, included package (github.com/jackc/pgx/v4/pgxpool).
pgtype has been spun off to a separate package (github.com/jackc/pgtype).
pgproto3 has been spun off to a separate package (github.com/jackc/pgproto3/v2).
Logical replication support has been spun off to a separate package (github.com/jackc/pglogrepl).
Lower level PostgreSQL functionality is now implemented in a separate package (github.com/jackc/pgconn).
Tests are now configured with environment variables.
Conn has an automatic statement cache by default.
Batch interface has been simplified.
QueryArgs has been removed.