Immunoglobulin G4 (IgG4) related disease is a localized or multisystem chronic inflammatory disease with characteristic pathological features. Several consensus criteria are available to aid in the diagnosis and its management. These criteria are highly specific, owing to their dependency on major histopathological criteria and laboratory investigations to diagnose IgG4-related diseases (IgG4-RD). We evaluated the lesions diagnosed as IgG4-RDs with histopathological scoring criteria based on histopathology, immunohistochemistry, and serology in the screening of IgG4-RDs. The lesions diagnosed as IgG4-RD during the study period were evaluated using the histopathological scoring criteria which include storiform fibrosis, obliterative phlebitis, plasmacytic infiltrates, and lymphoid follicles. Immunohistochemistry with IgG4 was performed on all these lesions. There were twelve patients with IgG4-RD involving the retroperitoneum, pancreas, lymph node, and nasal sinuses. Lesions involving the retroperitoneum showed all the characteristic histopathological findings of IgG4-RD. Storiform fibrosis and obliterative vasculitis were seen in 67% and 58% of the lesions respectively. Two patients (17%) lacked both storiform fibrosis and obliterative phlebitis. Additionally, lymphoid follicles were present in 58%, perineural infiltration in 33%, and tissue eosinophilia in 58% of individuals. Two patients with eosinophilic angiocentric fibrosis and a single patient with IgG4-RD associated with retroperitoneal mucinous cystadenoma were identified. IgG4-RD is a group of rare diseases that needs a multidisciplinary team approach for diagnosis. In our study, we report a rare association of IgG4-RD and retroperitoneal mucinous cystadenoma as well as rare lesions of IgG4-related eosinophilic angiocentric fibrosis. Clinicopathological and radiological correlation is crucial in the accurate diagnosis of IgG4-RD and to avoid overdiagnosis.