PostgreSQL Plugin for 4D
Pluggers Software has produced a very nice PostgreSQL plugin for 4th Dimension. It is based on libpq, so you'll find all the typical features you expect in a PostgreSQL client library including prepared statements, LISTEN/NOTIFY handling, and data type conversion. The library also includes handy row set management routines with calls similar to those used in 4D selection handling.
After evaluating the demo version for several days, I sent in my order along with some suggested improvements for handling NULL values in 4D. Shortly after my license arrived, I received a second email with a new version of the plugin attached. It included the features I had just requested.