The SD4 file extension is used by the Personal Information Manager (PIM) Smart Diary Suite software for the main database file. It is a kind of diary software, which can be used as an organizer, diary or health diary.
All entries are saved in the SD4 file. Note that the extension SD4 is the version 4 database file. Version 3 used the extension sdi, version 2 isd. Both file types can be imported from version 4 via 'Tools-> Import'.