As far as we know, both .drg and .dr4 files are CAD drawings from AllyCAD which should be your main choice for this conversion. If you do not have access to this particular CAD software, it will be pretty hard to perform this conversion without it.
Convert AllyCAD CAD design drawing to AllyCAD drawing.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert drg to dr4 on Windows
AllyCAD is a professional 2D and 3D CAD modeling software for civil engineering infrastructure design.
Convert from drg file
to dr4 file
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.