LibRaw 0.14.7 (latest stable release) and 0.15-Alpha1 (current alpha version) are able to process Ricoh GR IV DNGs without any problem. So, latest digiKam should be able to work with these files.
DarkTable, AFAIK, uses RawSpeed library for most RAW files processing and LibRaw as a fallback. You should reconfigure/recompile darktable to use LibRaw as a main RAW processing engine (do not know exact details, sorry).
LibRaw 0.14.7 (latest stable release) and 0.15-Alpha1 (current alpha version) are able to process Ricoh GR IV DNGs without any problem. So, latest digiKam should be able to work with these files.
DarkTable, AFAIK, uses RawSpeed library for most RAW files processing and LibRaw as a fallback. You should reconfigure/recompile darktable to use LibRaw as a main RAW processing engine (do not know exact details, sorry).