The small GTPases Rab are key regulators of intracellular membrane trafficking, from the formation of transport vesicles to their fusion with membranes. Rabs cycle between an inactive GDP-bound form and an active GTP-bound form that is able to recruit to membranes different set of downstream effectors directly responsible for vesicle formation, movement, tethering and fusion (By similarity). RAB4B mediates endosomal tethering and fusion through the interaction with RUFY1 and RAB14 (PubMed:20534812). Acts as a regulator of platelet alpha-granule release during activation and aggregation of platelets (By similarity). {ECO:0000250|UniProtKB:P20338, ECO:0000250|UniProtKB:Q91ZR1, ECO:0000269|PubMed:20534812}. This is the function of Ensembl gene identifier ENSG00000167578 (RAB4B, RAB4B, member RAS oncogene family).