A protein that is a translation product of the human CD4 gene or a 1:1 ortholog thereof. CD4 is an accessory protein for MHC class-II antigen/T-cell receptor interaction. It is the primary receptor for HIV-1. CD4 has four immunoglobulin-like domains in its extracellular region that share the same structure, but can differ in sequence. [ http://www.ncbi.nlm.nih.gov/pubmed/15326605 ]