Amino-acid permease involved in S-adenosylmethionine (SAM) transport and required for SAM-induced morphogenesis (PubMed:28028545). GAP4 is also able to transport arginine and thialysine, and thus probably also lysine (PubMed:21764911). Expression is under control of the CSY1 amino-acid sensor (PubMed:28028545). Expression is also regulated by HAP43, GCN2, GCN4 and SSN6 (PubMed:16215176, PubMed:15814841, PubMed:21592964). Induced during biofilm development (PubMed:22265407). Belongs to the amino acid-polyamine-organocation (APC) superfamily. YAT (TC 2.A.3.10) family.