Tumor immune regulation has been demonstrated in clinical studies using antibodies targeted to the B7/CD28 family. B7 homolog 4 (B7-H4) negatively regulates immune responses and is overexpressed in many types of human cancer, indicating that B7-H4 may be a potential target of cancer therapy. B7-H4 expression is affected by the microenvironment, and its presence has been reported in cancer tissues and immune cells. We found an upregulation of B7-H4 expression using comprehensive whole exome sequencing and gene expression profiling (project HOPE) launched by the Shizuoka Cancer Center based on tumor tissue samples from 1,058 cancer patients. We were successful in producing monoclonal antibodies for B7-H4 and demonstrated B7-H4 dimerization and rapid cell surface disappearance by antibody cross-linking in breast cancer cells, even under typical conditions. These observations may explain why antibody-dependent cellular cytotoxicity (ADCC) did not function in vivo on the B7-H4-expressing tumor cells. Unstable cell surface antigens are not suitable as targets for ADCC, and we therefore performed an indirect ADCC-redirecting T-cell cytotoxicity assay to study B7-H4 using polyclonal anti-mouse IgG antibody-mediated linking. Our results showed the possibility of targeting the B7-H4 molecule as a means of treating cancer.