The first explosion hit police headquarters in Cairo before Friday prayers. br br The nearby Islamic Art Museum was badly damaged.br br There was a second explosion a few hours later near a metro station in the Dokki district of Giza. br br Then a third blast near a police station in Al-Ahram area of Giza.br br The fourth explosion also in Giza - in front of a cinema.br br Al Jazeera's Richard Martin reports.