lib4d_sql is a C library that provides the required functions (connect, prepare, execute, etc.) for writing an SQL client for 4D databases. It uses the public 4D SQL protocol, which is supported by 4D products since version 11. It is already used by the PDO_4D Open Source project, and it might be used to provide others drivers for JDBC, ADO, or ActiveRecord.
PGJDBC-NG is an implementation of JDBC for the PostgreSQL server. It aims to support the complete JDBC 4.1. Specifically, it supports the advanced UDT features of JDBC that the mainstream driver lacks. It's implemented using an asynchronous I/O library (Netty), which allows it to support other advanced features like asynchronous notifications.