Fatty Acid Binding Protein 4 (FABP4), also known as A-FABP and aP2, is the predominant FABP found in adipocytes and is often used as a marker for adipocyte differentiation. It is also expressed in macrophages, dendritic cells, and endothelial cells. FABP4 is a key mediator of intracellular fatty acid transport and metabolism in adipose tissue. Its expression is regulated by multiple factors including fatty acids, PPAR gamma agonists, and Insulin, and its levels increase with lipolytic stimulation. It can increase the hydrolytic activity of Hormone-Sensitive Lipase and increase the production of proinflammatory cytokines and chemokines. FABP4 upregulation in adipocytes and macrophages is associated with the development of Insulin resistance, hypertriacylglycerolaemia, and atherosclerosis. Serum levels of FABP4 are elevated in obesity and metabolic syndrome, type 2 diabetes, HIV-associated lipodystrophy, polycystic ovary syndrome, nonalcoholic fatty liver disease, atherosclerosis, and acute ischaemic stroke. FABP4 is also overexpressed in multiple cancer types including ovarian, bladder, glioblastoma, and oral, and it may play a role in tumor progression.