A strain of HIV that enters and infects a host cell by binding to the CXCR4 receptor on the host cell. To enter a host cell, HIV must first attach to a CD4 receptor, then attach to either the CCR5 or CXCR4 receptor, and finally fuse its membrane with the host cell membrane. HIV is usually R5-tropic (uses CCR5) during the early stages of infection, but the virus may later switch to using either CXCR4 or both CCR5 and CXCR4.