Component of the BLOC-3 complex, a complex that acts as a guanine exchange factor (GEF) for RAB32 and RAB38, promotes the exchange of GDP to GTP, converting them from an inactive GDP-bound form into an active GTP-bound form. The BLOC-3 complex plays an important role in the control of melanin production and melanosome biogenesis and promotes the membrane localization of RAB32 and RAB38. Component of the biogenesis of lysosome-related organelles complex-3 (or BLOC-3), a heterodimer of HPS1 and HPS4. HPS4 and the BLOC-3 complex interact with the GTP-bound form of RAB9B but not with the GDP-bound form of RAB9B (By similarity). HPS4 and the BLOC-3 complex interact with the GTP-bound form of RAB9A but not with the GDP-bound form of RAB9A (PubMed:20048159, PubMed:26620560,). HPS4 does not interact RAB4A and RAB7A (PubMed:26620560). Highly expressed in heart, brain, liver and testis. Expressed at lower level in skeletal muscle. Defects in Hps4 are the cause of the light ear (le) mutant which exhibits hypopigmentation associated with defects of multiple cytoplasmic organelles, including melanosomes, lysosomes, and granular elements of platelets (PubMed:11836498).