The retinoblastoma protein pRB suppresses tumorigenesis largely through regulation of the E2F transcription factors. E2F4, the most abundant E2F protein, is thought to act in cooperation with pRB to restrain cell proliferation. In this study, we analyse how loss of E2f4 affects the tumorigenicity of pRB-deficient tissues. As Rb(-/-);E2f4(-/-) germline mice die in utero, we generated Rb(-/-);E2f4(-/-) chimeric animals to allow examination of adult tumor phenotypes. We found that loss of E2f4 had a differential effect on known Rb-associated neuroendocrine tumors. It did not affect thyroid and adrenal glands tumors but partially suppressed lung neuroendocrine hyperplasia. The most striking effect was in the pituitary where E2F4 loss delayed the development, and reduced the incidence, of Rb mutant tumors. This tumor suppression increased the longevity of the Rb(-/-);E2f4(-/-) chimeric animals allowing us to identify novel tumor types. We observed ganglionic neuroendocrine neoplasms, lesions not associated earlier with mutation of either Rb or E2f4. Moreover, a subset of the Rb(-/-);E2f4(-/-) chimeras developed either low- or high-grade carcinomas in the urothelium transitional epithelium supporting a key role for Rb in bladder cancer.