Here is a list of file extensions used by 4D on Windows:

.4DB Structure file data fork
.RSR Structure file resource fork
.4DC Compiled structure file data fork
.RSR Compiled structure file resource fork
.4DD Data file data fork
.4DR Data file resource fork
.4DS Data file segment
.4DE 4D Engine
.RSR 4D Engine resource fork
.HLP Windows help file
.TXT ASCII files created by 4D
.RES Workstation resource file
.REX Workstation local file
.ESR Proc.Ext file
.4DX Plug-in (data fork)
.RSR Plug-in (resource fork)
.4LK Hot link file
.4QR Quick report document
.4LB Labels document
.4ST SAVE SET document
.4VR SAVE VARIABLE document
.4UG User & Groups set
.PTH Path name document
.4FI ASCII filter document (both export and import maps)
.4FR Apply formula document
.4DF Search document
.TFR Formula (search, sort, etc.)
.CMP Runtime libraries used by a compiled structure
.SEL Named selection paged to disk
.DLL Dynamic Linked Libraries used by 4D

.4BP 4D Backup project file
.4DL 4D Backup log file
.4BK 4D Backup file
.4BR 4D Backup file description of archive contents
.4BL 4D Backup log file backup

.4CT 4D Chart document
.PCT 4D Chart document exported in PICT format

.4DW 4D Draw document or template
.PCT 4D Draw document saved in PICT format
.PNT 4D Draw document saved in MacPaint format

.4SP 4D Calc document or template
.SLK 4D Calc document saved in SYLK format
.TXT 4D Calc document saved as ASCII file

.4WR 4D Write v6 document or template
.4W7 4D Write 6.5 document
.4WT 4D Write 6.5 template
.4WS 4D Write style sheet
.RTF RTF document saved from 4D Write
.HTML HTML document saved from 4D Write
.TXT Text document saved from 4D Write

.PRJ 4D Compiler project
.SYM 4D Compiler symbol table
.TYP 4D Compiler typing table
.EXE 4D application, or a compiled and merged 4D application
.ERC Created by 4D Compiler, used for reading .ERR file inside 4D
.ERR 4D Compiler error file.

.FON ASI System font
.MAP Font Map