May act as a GTPase-activating protein for RAB2A, RAB8A, RAB10 and RAB14. Isoform 2 promotes insulin-induced glucose transporter SLC2A4/GLUT4 translocation at the plasma membrane, thus increasing glucose uptake. {ECO:0000269|PubMed:15971998, ECO:0000269|PubMed:18771725, ECO:0000269|PubMed:22908308}. This is the function of TBC1D4 (TBC1 domain family member 4, ENSG00000136111).