CD4(+) T cells are crucial in shaping response and resistance to immunotherapy. To enhance our understanding of their multifaceted functions, we developed copper-64-radiolabeled nanobodies targeting the human CD4 receptor ((64)Cu-CD4-Nb1) for positron emission tomography (PET). In human CD4-receptor knock-in mice, (64)Cu-CD4-Nb1 specifically accumulated in different orthotopic tumors, correlating with histological CD4(+) cell densities. Based on intratumoral CD4(+) cell distribution patterns within the core and periphery, we distinguished responders to combined αPD-1/4-1BB antibodies early on-treatment. CD4-PET identified resistance to αPD-1 monotherapy, which was mitigated by adding regulatory T cell-depleting α4-1BB antibodies. Patients with early-stage non-small cell lung cancer who relapsed after neoadjuvant αPD-L1 therapy revealed low CD4(+) T cell densities in the tumor core. In human and mouse tumor tissues, regulatory T cells correlated with CD4(+) cell densities. Thus, visualizing the spatial distribution patterns of CD4(+) cells by PET offers mechanistic insights into CD4-mediated therapy efficacy, with great potential for guiding combinatorial immunotherapies in patients with cancer.