There is only one file type that uses the .NS4 extension:
Lotus Notes 4 Database
Database file created by Lotus Notes 4.x; contain a list of contacts, text notes, e-mail messages, calendars, and other information; stores structured data for personal and business organizational management.
"Domino," the Notes server program, was released in 1996 along with Notes 4.5. Many following updates were only release for Windows, but Lotus Notes 7.0.2 was released for Mac OS X in 2006.
Updated: July 21, 2006