There is only one file type that uses the .TG4 extension:
Tiled Group 4 Raster Image File
Raster image format used by Bentley Systems software, also known as a "Type 24" file; saved as a 1-bit tiled image file compressed with RLE compression; supports multiple resolutions.
TG4 is the native format used by ImageCenter and Image Machines software.
Updated: January 26, 2009