Earlier reports had put the death toll at six. Hospital officials said the casualty count included 29 injuries.
A spokesman for the Pakistani Taliban condemned the blast inside the Chishtia Hanafia Ganj Ali Khan mosque near a major bazaar, telling Dawn News the organization opposed attacks on public places.
Officials believe 13 pounds of explosives were planted in the front row of the congregation and detonated by remote control, Russia's RIA Novosti news agency said.
The hills around Peshawar have been the scene of ongoing clashes between the military and Taliban and al-Qaida fighters. The conflict has spilled into the city in the form of frequent bombings.