The function of RAPGEF4 (Rap guanine nucleotide exchange factor 4, ENSG00000091428) is as follows. Guanine nucleotide exchange factor (GEF) for RAP1A, RAP1B and RAP2A small GTPases that is activated by binding cAMP. Does not seem to activate RAB3A. Involved in cAMP-dependent, PKA-independent exocytosis through interaction with RIMS2 (By similarity). {ECO:0000250, ECO:0000269|PubMed:10777494, ECO:0000269|PubMed:9856955}.