In Korea, a nationwide surveillance program has been implemented to identify highly pathogenic avian influenza virus since 2003. Avian influenza viruses (AIVs) belonging to one of the most common haemagglutinin subtypes, H4, were isolated from two domestic ducks and 52 wild birds in 2004-2010. These H4 AIVs could be further classified into three neuraminidase subtypes: H4N6 (94.4%), H4N2 (3.7%), and H4N3 (1.9%). Phylogenetic analysis revealed that the H4 AIVs had a variety of genetic constellations, with at least nine different genotypes represented. We assessed the pathogenicity of H4 viruses in quails, domestic ducks, and mice. None of the H4 AIVs induced clinical signs in quails or domestic ducks. Viral shedding in quail was relatively high, which were recovered up to 5-7 days post inoculation (DPI) in oropharyngeal (OP) swabs but viruses replicated poorly in domestic ducks. Quail may act as an intermediate host in which AIVs are amplified and transmitted to other species. In mice, all of the AIVs were efficiently recovered to relatively high titres from the lungs up to 7 DPI, demonstrating the potential for AIVs to directly infect mice without prior adaptation. None of the AIVs induced clinical signs nor were any lethal to infected mice. However, there was significant loss of body weight in mice infected with viruses of duck origin. We suggest that the active surveillance of influenza viruses needs to be enhanced in domestic poultry as well as in wild birds, and that it should include assessment of pathogenicity in animal models.