DEP4 file extension - Data Exchange Protocol detailing file
What is dep4 file? How to open dep4 files?
File type specification:
The DEP1 file extension is used for one of the Data Exchange Protocol files. The data exchange protocols (DEP) define logical sets of the complete data at defined stages in the process, from analysis through design and fabrication. A *.dep4 file contains detailing data.
Seems like it was used in the past in some 3D engineering software.