Retinol-binding protein that mediates retinol transport in blood plasma (PubMed:5541771). Delivers retinol from the liver stores to the peripheral tissues (Probable). Transfers the bound all-trans retinol to STRA6, that then facilitates retinol transport across the cell membrane (PubMed:22665496). {ECO:0000269|PubMed:22665496, ECO:0000305, ECO:0000305|PubMed:5541771}. This is the function of Ensembl gene identifier ENSG00000138207 (RBP4, retinol binding protein 4).