Local database file (4th Dimension)
REX is a file extension for a local database file used with 4th Dimension Software. REX files contain a database used cross-platform, web-enabled solutions. REX files can be opened with 4th Dimension software, created by 4D Inc.
Learn more about .REX files:
Visit the 4D web page.
Read the Database FAQ at ITKnowledgeExchange.com.