ΔE 6.6, close: similar, with a visible difference side by side. Russian Green 4BO is a little lighter than Dark Feldgrau.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Russian Green 4BO is a little lighter than Dark Feldgrau.
The same difference applies the other way round: Dark Feldgrau is a little darker.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Dark Feldgrau on the colour wheel See Russian Green 4BO on the colour wheel
All matches: Dark Feldgrau equivalents · Russian Green 4BO equivalents