Patients infected with hepatitis C virus genotype 4 (HCV-4) respond to interferon alpha (IFN) as poorly as those infected with genotype 1. However, there is no information on the viral dynamics of HCV-4. Interferon-ribavirin treatment was administered to untreated patients infected with HCV-4. Viral load was assessed with Versant 3.0. Viral dynamics parameters were estimated based on the bi-phasic model for HCV during IFN treatment. Viral kinetics of HCV-4 follow a bi-phasic decline pattern also. The mean effectiveness of IFN in blocking production of HCV-4 was a decline of 77.8% during the first day of treatment. The half-life of free virions was estimated at 3.5 h and that of infected cells from 1.9 to over 70 days. The viral dynamics parameters of HCV-4 appear similar to those of HCV-1 and slower than those of HCV-2. HCV-4 infected patients should be grouped with those with HCV-1 when therapeutic schemes are considered in relation to genotype.