It seems that LibRAW is not supporting DNG raw from Ricoh GR Digital IV. At least I was not able to recompress it with KIPI plugin's DNG Converter and also failed to open in DarkTable so I assume that lacking support in LibRaw is the culprit. Here are sample raw files for reference: http://www.photographyblog.com/reviews/ricoh_gr_digital_iv_review/sample... Please let me know If I can do anything to help.
LibRaw
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LibRaw 0.14.7 (latest stable
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).
-- Alex Tutubalin @LibRaw LLC