Phosphatidylglycerophosphatase involved in the biosynthesis of cardiolipin (CL), a unique dimeric phosphoglycerolipid predominantly present in mitochondrial membranes and which has important functions for cellular energy metabolism, mitochondrial dynamics and the initiation of apoptotic pathways. 1,2-diacyl-sn-glycero-3-phospho-(1'-sn-glycero-3'-phosphate) + H2O = 1,2-diacyl-sn-glycero-3-phospho-(1'-sn-glycerol) + phosphate Phospholipid metabolism; phosphatidylglycerol biosynthesis; phosphatidylglycerol from CDP-diacylglycerol: step 2/2. Belongs to the GEP4 family.