Catalyzes the conversion of GDP-D-mannose to GDP-4-dehydro-6-deoxy-D-mannose. GDP-alpha-D-mannose = GDP-4-dehydro-alpha-D-rhamnose + H2O Nucleotide-sugar biosynthesis; GDP-L-fucose biosynthesis via de novo pathway; GDP-L-fucose from GDP-alpha-D-mannose: step 1/2. Homotetramer. Binds to GER1. Expressed in roots, flowers, siliques, leaves and stems. Not expressed in the root meristem and the proximal part of the elongation zone, or in emerging lateral roots. Expressed in trichomes and guard cells, and in pollen just before anthesis. Belongs to the NAD(P)-dependent epimerase/dehydratase family. GDP-mannose 4,6-dehydratase subfamily.