CAIRO — Antigovernment protesters fought street battles with police in Cairo and other cities on Sunday, the fourth anniversary of the country’s 2011 uprising, as clashes left at least 15 people dead and dozens injured. Two others died when an explosive device they were planting under a high-voltage tower in the Nile Delta exploded prematurely, according to security officials. Most of the deaths took place in Cairo’s eastern Matariyah district — an Islamist stronghold where police used tear gas and birdshot to disperse supporters of the outlawed Muslim Brotherhood group armed with firebombs and rocks. The violence underscored the continued turmoil roiling the Arab world’s most populous nation four years after the 18-day uprising that toppled autocrat Hosni Mubarak.