The databases which work in 6.2 or 6.3 should work in 6.4.
The choice of the base depends first of all on what you needs. Large or the small base. Given on the disc or in the RAM 
memory.
Look:
Raima Database Manager (RDM) Embedded => http://www.raima.com/products/rdm-embedded/specification/

McObject’s embedded database - eXtremeDB  => http://www.mcobject.com/embedded_database_products

MySQL 5.0  - http://dev.mysql.com/get/Downloads/MySQL-5.0/mysql-5.0.67-nto-qnx6.2.1-i386.tar.gz  - I try it in 6.4 work 
very well.

Empress RDBMS  => http://www2.empress.com/qnx/empqnx.html

Oracle Berkeley DB  => http://www.oracle.com/technology/products/berkeley-db/db/index.html  very good work on 6.4

DB2® Everyplace® Enterprise Edition => http://publib.boulder.ibm.com/infocenter/db2e/v9r1/index.jsp?topic=/com.ibm.
db2e.doc/dbeint0316.html

SQLite => http://www.sqlite.org/  work on QNX 6.3/6.4