Equine CCL4 (C-C motif chemokine ligand 4, also known as MIP-1β, macrophage inflammatory protein-1 beta) is a pro-inflammatory chemokine that regulates recruitment and activation of immune cells during inflammatory and infectious responses in horses (Equus caballus). CCL4 primarily signals through the CCR5 receptor, promoting chemotaxis of monocytes, macrophages, dendritic cells, natural killer (NK) cells, and activated T lymphocytes to sites of infection or tissue injury. In horses, CCL4 is produced by activated macrophages, dendritic cells, epithelial cells, and lymphocytes in response to inflammatory stimuli such as microbial components and cytokines including TNF-α and IL-1β. CCL4-mediated immune cell recruitment contributes to host defense in diseases such as equine influenza virus infection, bacterial pneumonia, and other respiratory pathogens, where leukocyte trafficking to lung tissue helps coordinate antiviral and antibacterial immune responses. It may also play roles in musculoskeletal inflammation and joint disease, where infiltration of immune cells contributes to inflammatory signaling and tissue remodeling. While CCL4-driven leukocyte migration supports pathogen clearance and immune activation, excessive or prolonged expression may contribute to inflammatory tissue damage. As a regulator of immune cell trafficking and inflammatory signaling, equine CCL4 is important in studies of host–pathogen interactions, respiratory disease, and inflammatory mechanisms affecting equine health and performance.