4.0 adds transaction support ("BEGIN" and "COMMIT") and IIRC another database file type. It is supposed to also be generally faster. IOW, if 3.23 gives you everything, then 4.0 is probably not worth it.

PHP 4.x has support for PostGres. I haven't used it but it is in the manual. In RedHat, the PostGreSQL support in PHP is an extra RPM; but so is MySQL.

Wade.